Coated steel member, coated steel sheet, and methods for producing same

ABSTRACT

This coated steel member includes: a steel sheet substrate having a predetermined chemical composition; and a coating formed on a surface of the steel sheet substrate and containing Al and Fe, in which the coating has a low Al content region having an Al content of 3 mass % or more and less than 30 mass % and a high Al content region formed on a side closer to a surface than the low Al content region and having an Al content of 30 mass % or more, a maximum C content of the high Al content region is 25% or less of a C content of the steel sheet substrate, a maximum C content of the low Al content region is 40% or less of the C content of the steel sheet substrate, and a maximum C content in a range from an interface between the steel sheet substrate and the coating to a depth of 10 μm on a side of the steel sheet substrate is 80% or less of the C content of the steel sheet substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of copending application Ser. No.17/298,369, filed on May 28, 2021, which is the National Phase under 35U.S.C. § 371 of International Application No. PCT/JP2020/014581, filedon Mar. 30, 2022, which claims the benefit under 35 U.S.C. § 119(a) toPatent Application No. 2019-068658, filed in Japan on Mar. 29, 2019, allof which are hereby expressly incorporated by reference into the presentapplication.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a coated steel member, a coated steelsheet, and methods for producing the same.

Priority is claimed on Japanese Patent Application No. 2019-068658,filed Mar. 29, 2019, the content of which is incorporated herein byreference.

BACKGROUND ART

In the field of steel sheets for a vehicle, against the background oftightening of recent environmental regulations and collision safetystandards, the application of steel sheets having high tensile strength(high strength steel sheet) has expanded in order to improve both thefuel economy and collision safety. However, the press formability of asteel sheet decreases with high-strengthening, which makes it difficultto produce a product having a complex shape.

Specifically, the ductility of the steel sheet decreases with thehigh-strengthening, and there is a problem that the steel sheet isfractured at a highly processed portion in a case of being processedinto a complex shape. In addition, with the high-strengthening of thesteel sheet, the residual stress after processing causes springback andwall warpage, which also causes a problem that the dimensional accuracyis deteriorated. Therefore, it is not easy to perform press forming on asteel sheet having high strength, particularly a tensile strength of 780MPa or more, into a product having a complex shape. Roll forming ratherthan press forming makes it easier to process a high strength steelsheet, but the application thereof is limited to components having auniform cross section in the longitudinal direction.

Therefore, in recent years, for example, as disclosed in PatentDocuments 1 to 3, a hot stamping technique has been adopted as atechnique for press-forming a material that is difficult to form, suchas a high strength steel sheet. The hot stamping technique is a hotforming technique of heating a material to be subjected to forming andthen forming the material.

In this technique, the material is formed after being heated. Therefore,the steel is soft at the time of forming and has good formability.Accordingly, even a high strength steel sheet can be accurately formedinto a complex shape. Furthermore, in the hot stamping technique, sincehardening is performed simultaneously with forming by a press die, thesteel member after the forming has sufficient strength.

For example, according to Patent Document 1, it is disclosed that it ispossible to impart a tensile strength of 1400 MPa or more to a steelmember after forming by a hot stamping technique.

On the other hand, a goal of higher fuel economy is set in variouscountries around the world. Along with this, a reduction in the weightof the vehicle body has been examined, and higher strength is requiredfor a steel member used for the vehicle body. For example, there is aneed for a steel member that has a strength more than 1.5 GPa, which isthe strength of a member generally used as a hot stamping member atpresent.

In addition, as described above, vehicles are also required to havecollision safety. The collision safety of a vehicle is evaluated by thecrushing strength and absorbed energy in a collision test of the entirevehicle body or some members. In particular, since the crushing strengthlargely depends on the material strength, the demand for ultrahighstrength steel members as vehicle members dramatically increases.

However, in general, the steel members decrease in fracture toughnessand deformability with the high-strengthening. Therefore, the steelmember is fractured prematurely during crushing due to a collision or isfractured at a portion on which deformation is likely to beconcentrated, so that there are cases where crushing strength suitablefor the material strength is not exhibited, and a sufficient absorbedenergy cannot be obtained. Therefore, in order to improve the collisionsafety of vehicles, it is required to improve not only the materialstrength but also the fracture toughness and deformability, that is, thetoughness and bendability of the steel members used.

When the steel member has a higher strength than that of a hot stampingmember in the related art, specifically, a tensile strength of more than1.5 GPa, the bendability and toughness are further deteriorated.Therefore, in order to apply a high strength steel member having atensile strength of more than 1.5 GPa to a vehicle body, there is a needfor a technique for providing a steel member having toughness andbendability higher than in the related art and exhibiting sufficientabsorbed energy even in the event of a collision accident.

Regarding a high strength steel having a tensile strength of more than1.5 GPa, for example, Patent Document 2 discloses a press-formed articlehaving excellent toughness and a tensile strength of 1.8 GPa or more,which is hot press-formed. Patent Document 3 discloses a steel having atensile strength as extremely high as 2.0 GPa or more, and furtherhaving good toughness and ductility. Patent Document 4 discloses a steelhaving a tensile strength as high as 1.8 GPa or more and further havinggood toughness. Patent Document 5 discloses a steel having a tensilestrength as extremely high as 2.0 GPa or more and further having goodtoughness.

However, Patent Documents 2 to 5 do not describe techniques regardingbendability, and cannot sufficiently meet higher demands in the use ofhigh strength steels having a tensile strength of more than 1.5 GPa asvehicle members in some cases.

Regarding bendability, for example, Patent Documents 6 to 10 disclose ahot-stamping formed body having excellent bendability. However, PatentDocuments 6 to 10 do not describe techniques regarding toughness, andcannot sufficiently meet higher demands in the use of high strengthsteels having a tensile strength of more than 1.5 GPa as vehicle membersin some cases.

PRIOR ART DOCUMENT Patent Document

[Patent Document 1] Japanese Unexamined Patent Application, FirstPublication No. 2002-102980

[Patent Document 2] Japanese Unexamined Patent Application, FirstPublication No. 2012-180594

[Patent Document 3] Japanese Unexamined Patent Application, FirstPublication No. 2012-1802

[Patent Document 4] PCT International Publication No. WO2015/182596

[Patent Document 5] PCT International Publication No. WO2015/182591

[Patent Document 6] PCT International Publication No. WO2015/033177

[Patent Document 7] PCT International Publication No. WO2018/151333

[Patent Document 8] PCT International Publication No. WO2018/151330

[Patent Document 9] PCT International Publication No. WO2018/151332

[Patent Document 10] PCT International Publication No. WO2018/151325

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The present invention has been made to solve the above problems, and anobject thereof is to provide a coated steel member having high tensilestrength and excellent toughness and bendability, a coated steel sheetsuitable as a material for the steel member, and methods for producingthe same.

Means for Solving the Problem

The gist of the present invention is a coated steel member, a coatedsteel sheet, and methods for producing the same as follows. Hereinafter,a steel sheet in which the surface is not subjected to a coating, whichis a material of the coated steel sheet, is simply referred to as a“steel sheet”.

(1) A coated steel member according to an aspect of the presentinvention includes: a steel sheet substrate containing, as a chemicalcomposition, by mass %, C: 0.25% to 0.65%, Si: 0.10% to 2.00%, Mn: 0.30%to 3.00%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, Ti:0.010% to 0.100%, B: 0.0005% to 0.0100%, Nb: 0.02% to 0.10%, Mo: 0% to1.00%, Cu: 0% to 1.00%, Cr: 0% to 1.00%, Ni: 0% to 1.00%, V: 0% to1.00%, Ca: 0% to 0.010%, Al: 0% to 1.00%, Sn: 0% to 1.00%, W: 0% to1.00%, Sb: 0% to 1.00%, REM: 0% to 0.30%, and a remainder of Fe andimpurities; and a coating formed on a surface of the steel sheetsubstrate and containing Al and Fe, in which the coating has a low Alcontent region having an Al content of 3 mass % or more and less than 30mass % and a high Al content region formed on a side closer to a surfacethan the low Al content region and having an Al content of 30 mass % ormore, a maximum C content of the high Al content region is 25% or lessof a C content of the steel sheet substrate, a maximum C content of thelow Al content region is 40% or less of the C content of the steel sheetsubstrate, and a maximum C content in a range from an interface betweenthe steel sheet substrate and the coating to a depth of 10 μm on a sideof the steel sheet substrate is 80% or less of the C content of thesteel sheet substrate.

(2) In the coated steel member according to (1), the steel sheetsubstrate may contain, as the chemical composition, Cr: 0.05% to 1.00%,and a maximum Cr content in the high Al content region may be 80% ormore of a Cr content of the steel sheet substrate.

(3) A coated steel sheet according to another aspect of the presentinvention includes: a steel sheet containing, as a chemical composition,by mass %, C: 0.25% to 0.65%, Si: 0.10% to 2.00%, Mn: 0.30% to 3.00%, P:0.050% or less, S: 0.0100% or less, N: 0.010% or less, Ti: 0.010% to0.100%, B: 0.0005% to 0.0100%, Nb: 0.02% to 0.10%, Mo: 0% to 1.00%, Cu:0% to 1.00%, Cr: 0% to 1.00%, Ni: 0% to 1.00%, V: 0% to 1.00%, Ca: 0% to0.010%, Al: 0% to 1.00%, Sn: 0% to 1.00%, W: 0% to 1.00%, Sb: 0% to1.00%, REM: 0% to 0.30%, and a remainder of Fe and impurities; and acoating formed on a surface of the steel sheet and containing Al, inwhich the coating includes a lower layer being present on a side of thesteel sheet and containing 3 mass % or more and less than 70 mass % ofAl and an upper layer containing 70 mass % or more and 95 mass % or lessof Al, the lower layer contains Cr in an amount of 1.2 times or more aCr content in the steel sheet by mass %, or the upper layer contains Siand Ni in a total amount of 5.0 mass % or more and 30.0 mass % or less,and a maximum C content in a range from an interface between the steelsheet and the coating to a depth of 20 μm on a side of the steel sheetis 80% or less of an average C content in an overall sheet thickness ofthe steel sheet.

(4) In the coated steel sheet according to (3), the steel sheet maycontain, as the chemical composition, Cr: 0.05% to 1.00%, and in thecoating, the lower layer may contain Cr in an amount of 1.2 times ormore the Cr content in the steel sheet, and the upper layer may containSi and Ni in a total amount of 5.0 mass % or more and 30.0 mass % orless.

(5) A method for producing a coated steel sheet according to anotheraspect of the present invention, includes: a slab preparation step ofmelting a steel containing, as a chemical composition, by mass %, C:0.25% to 0.65%, Si: 0.10% to 2.00%, Mn: 0.30% to 3.00%, P: 0.050% orless, S: 0.0100% or less, N: 0.010% or less, Ti: 0.010% to 0.100%, B:0.0005% to 0.0100%, Nb: 0.02% to 0.10%, Mo: 0% to 1.00%, Cu: 0% to1.00%, Cr: 0% to 1.00%, Ni: 0% to 1.00%, V: 0% to 1.00%, Ca: 0% to0.010%, Al: 0% to 1.00%, Sn: 0% to 1.00%, W: 0% to 1.00%, Sb: 0% to1.00%, REM: 0% to 0.30%, and a remainder of Fe and impurities, andcasting to obtain a slab; a hot rolling step of hot-rolling the slab toobtain a hot-rolled steel sheet; a coiling step of coiling thehot-rolled steel sheet; a hot-rolled sheet annealing step of annealingthe hot-rolled steel sheet after the coiling in an atmosphere containing80% or more of nitrogen at 450° C. to 800° C. for 5 hours or longer; asnecessary, a cold rolling step of descaling the hot-rolled steel sheetand cold-rolling the hot-rolled steel sheet to obtain a cold-rolledsteel sheet; as necessary, an annealing step of annealing the hot-rolledsteel sheet or the cold-rolled steel sheet to obtain an annealed steelsheet; and a coating step of forming an Al-based coating on thehot-rolled steel sheet, the cold-rolled steel sheet, or the annealedsteel sheet to obtain a coated steel sheet.

(6) A method for producing a coated steel sheet according to anotheraspect of the present invention, includes: a slab preparation step ofmelting a steel containing, as a chemical composition, by mass %, C:0.25% to 0.65%, Si: 0.10% to 2.00%, Mn: 0.30% to 3.00%, P: 0.050% orless, S: 0.0100% or less, N: 0.010% or less, Ti: 0.010% to 0.100%, B:0.0005% to 0.0100%, Nb: 0.02% to 0.10%, Mo: 0% to 1.00%, Cu: 0% to1.00%, Cr: 0% to 1.00%, Ni: 0% to 1.00%, V: 0% to 1.00%, Ca: 0% to0.010%, Al: 0% to 1.00%, Sn: 0% to 1.00%, W: 0% to 1.00%, Sb: 0% to1.00%, REM: 0% to 0.30%, and a remainder of Fe and impurities, andcasting to obtain a slab; a hot rolling step of hot-rolling the slab toobtain a hot-rolled steel sheet; a coiling step of coiling thehot-rolled steel sheet; as necessary, annealing the hot-rolled steelsheet; as necessary, a cold rolling step of descaling the hot-rolledsteel sheet and cold-rolling the hot-rolled steel sheet to obtain acold-rolled steel sheet; an annealing step of annealing the hot-rolledsteel sheet or the cold-rolled steel sheet to obtain an annealed steelsheet in an atmosphere having a dew point of 1° C. or higher and in atemperature range of 700° C. to 950° C.; and a coating step of formingan Al-based coating on a surface of the annealed steel sheet to obtain acoated steel sheet by immersing the annealed steel sheet in a platingbath containing Si and Ni in a total amount of 7.0 to 30.0 mass % and aremainder of Al and impurities.

(7) A method for producing a coated steel sheet according to anotheraspect of the present invention, includes: a slab preparation step ofmelting a steel containing, as a chemical composition, by mass %, C:0.25% to 0.65%, Si: 0.10% to 2.00%, Mn: 0.30% to 3.00%, P: 0.050% orless, S: 0.0100% or less, N: 0.010% or less, Ti: 0.010% to 0.100%, B:0.0005% to 0.0100%, Nb: 0.02% to 0.10%, Mo: 0% to 1.00%, Cu: 0% to1.00%, Cr: 0% to 1.00%, Ni: 0% to 1.00%, V: 0% to 1.00%, Ca: 0% to0.010%, Al: 0% to 1.00%, Sn: 0% to 1.00%, W: 0% to 1.00%, Sb: 0% to1.00%, REM: 0% to 0.30%, and a remainder of Fe and impurities, andcasting to obtain a slab; a hot rolling step of hot-rolling the slab toobtain a hot-rolled steel sheet; a coiling step of coiling thehot-rolled steel sheet; a hot-rolled sheet annealing step of annealingthe hot-rolled steel sheet after the coiling in an atmosphere containing80% or more of nitrogen at 450° C. to 800° C. for 5 hours or longer; asnecessary, a cold rolling step of descaling the hot-rolled steel sheetand cold-rolling the hot-rolled steel sheet to obtain a cold-rolledsteel sheet; an annealing step of annealing the hot-rolled steel sheetor the cold-rolled steel sheet to obtain an annealed steel sheet in anatmosphere having a dew point of 1° C. or higher and in a temperaturerange of 700° C. to 950° C.; and a coating step of forming an Al-basedcoating on a surface of the annealed steel sheet to obtain a coatedsteel sheet by immersing the annealed steel sheet in a plating bathcontaining Si and Ni in a total amount of 7.0 to 30.0 mass % and aremainder of Al and impurities.

(8) A method for producing a coated steel member according to anotheraspect of the present invention, includes: a slab preparation step ofmelting a steel containing, as a chemical composition, by mass %, C:0.25% to 0.65%, Si: 0.10% to 2.00%, Mn: 0.30% to 3.00%, P: 0.050% orless, S: 0.0100% or less, N: 0.010% or less, Ti: 0.010% to 0.100%, B:0.0005% to 0.0100%, Nb: 0.02% to 0.10%, Mo: 0% to 1.00%, Cu: 0% to1.00%, Cr: 0% to 1.00%, Ni: 0% to 1.00%, V: 0% to 1.00%, Ca: 0% to0.010%, Al: 0% to 1.00%, Sn: 0% to 1.00%, W: 0% to 1.00%, Sb: 0% to1.00%, REM: 0% to 0.30%, and a remainder of Fe and impurities, andcasting to obtain a slab; a hot rolling step of hot-rolling the slab toobtain a hot-rolled steel sheet; a coiling step of coiling thehot-rolled steel sheet; a hot-rolled sheet annealing step of annealingthe hot-rolled steel sheet after the coiling in an atmosphere containing80% or more of nitrogen at 450° C. to 800° C. for 5 hours or longer; asnecessary, a cold rolling step of descaling the hot-rolled steel sheetand cold-rolling the hot-rolled steel sheet to obtain a cold-rolledsteel sheet; as necessary, an annealing step of annealing the hot-rolledsteel sheet or the cold-rolled steel sheet to obtain an annealed steelsheet; a coating step of forming an Al-based coating on the hot-rolledsteel sheet, the cold-rolled steel sheet, or the annealed steel sheet toobtain a coated steel sheet; and a heat treatment step of heating thecoated steel sheet to an Ac₃ point to (Ac₃ point+300)° C. at atemperature rising rate of 1.0 to 1000° C./s, and thereafter cooling thecoated steel sheet to an Ms point or lower at an upper critical coolingrate or more.

(9) A method for producing a coated steel member according to anotheraspect of the present invention, includes: a slab preparation step ofmelting a steel containing, as a chemical composition, by mass %, C:0.25% to 0.65%, Si: 0.10% to 2.00%, Mn: 0.30% to 3.00%, P: 0.050% orless, S: 0.0100% or less, N: 0.010% or less, Ti: 0.010% to 0.100%, B:0.0005% to 0.0100%, Nb: 0.02% to 0.10%, Mo: 0% to 1.00%, Cu: 0% to1.00%, Cr: 0% to 1.00%, Ni: 0% to 1.00%, V: 0% to 1.00%, Ca: 0% to0.010%, Al: 0% to 1.00%, Sn: 0% to 1.00%, W: 0% to 1.00%, Sb: 0% to1.00%, REM: 0% to 0.30%, and a remainder of Fe and impurities, andcasting to obtain a slab; a hot rolling step of hot-rolling the slab toobtain a hot-rolled steel sheet; a coiling step of coiling thehot-rolled steel sheet; as necessary, a hot-rolled sheet annealing stepof annealing the hot-rolled steel sheet; as necessary, a cold rollingstep of descaling the hot-rolled steel sheet and cold-rolling thehot-rolled steel sheet to obtain a cold-rolled steel sheet; an annealingstep of annealing the hot-rolled steel sheet or the cold-rolled steelsheet to obtain an annealed steel sheet in an atmosphere having a dewpoint of 1° C. or higher and in a temperature range of 700° C. to 950°C.; a coating step of forming an Al-based coating on a surface of theannealed steel sheet to obtain a coated steel sheet by immersing theannealed steel sheet in a plating bath containing Si and Ni in a totalamount of 7.0 to 30.0 mass % and a remainder of Al and impurities; and aheat treatment step of heating the coated steel sheet to an Ac₃ point to(Ac₃ point+300)° C. at a temperature rising rate of 1.0 to 1000° C./s,and thereafter cooling the coated steel sheet to an Ms point or lower atan upper critical cooling rate or more.

(10) A method for producing a coated steel member according to anotheraspect of the present invention, includes: a slab preparation step ofmelting a steel containing, as a chemical composition, by mass %, C:0.25% to 0.65%, Si: 0.10% to 2.00%, Mn: 0.30% to 3.00%, P: 0.050% orless, S: 0.0100% or less, N: 0.010% or less, Ti: 0.010% to 0.100%, B:0.0005% to 0.0100%, Nb: 0.02% to 0.10%, Mo: 0% to 1.00%, Cu: 0% to1.00%, Cr: 0% to 1.00%, Ni: 0% to 1.00%, V: 0% to 1.00%, Ca: 0% to0.010%, Al: 0% to 1.00%, Sn: 0% to 1.00%, W: 0% to 1.00%, Sb: 0% to1.00%, REM: 0% to 0.30%, and a remainder of Fe and impurities, andcasting to obtain a slab; a hot rolling step of hot-rolling the slab toobtain a hot-rolled steel sheet; a coiling step of coiling thehot-rolled steel sheet; a hot-rolled sheet annealing step of annealingthe hot-rolled steel sheet after the coiling in an atmosphere containing80% or more of nitrogen at 450° C. to 800° C. for 5 hours or longer; asnecessary, a cold rolling step of descaling the hot-rolled steel sheetand cold-rolling the hot-rolled steel sheet to obtain a cold-rolledsteel sheet; an annealing step of annealing the hot-rolled steel sheetor the cold-rolled steel sheet to obtain an annealed steel sheet in anatmosphere having a dew point of 1° C. or higher and in a temperaturerange of 700° C. to 950° C.; a coating step of forming an Al-basedcoating on a surface of the annealed steel sheet to obtain a coatedsteel sheet by immersing the annealed steel sheet in a plating bathcontaining Si and Ni in a total amount of 7.0 to 30.0 mass % and aremainder of Al and impurities; and a heat treatment step of heating thecoated steel sheet to an Ac₃ point to (Ac₃ point+300)° C. at atemperature rising rate of 1.0 to 1000° C./s, and thereafter cooling thecoated steel sheet to an Ms point or lower at an upper critical coolingrate or more.

Effects of the Invention

According to the above aspects of the present invention, it is possibleto provide a coated steel member having high tensile strength andexcellent toughness and bendability, a coated steel sheet, and methodsfor producing the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an example of a coated steel memberaccording to the present embodiment.

FIG. 2 is a schematic view showing an example of a coated steel sheetaccording to the present embodiment.

EMBODIMENTS OF THE INVENTION

In order to obtain a coated steel member having high tensile strengthand excellent toughness and bendability, the present inventorsinvestigated the effect of chemical compositions and structures on theseproperties. As a result, the following findings were obtained.

Most of the steel sheets as materials for hot stamping members that aregenerally produced are coated steel sheets in which the surface issubjected to an aluminum plating (Al plating) or zinc plating (Znplating) having excellent corrosion resistance. When hot stamping isperformed on these coated steel sheets, the reaction of the alloy on thesurface progresses, whereby a coated steel member having an Al—Fe-basedcoating or a Zn—Fe-based coating can be obtained. Most of the commonlyused steel sheets showing a tensile strength of 1.5 GPa class after hotstamping have similar chemical compositions and contain about 0.20 mass% of C, and strength after the hot stamping is secured by C.

(a) In order to achieve a further reduction in the weight of the vehiclebody, the present inventors conducted a detailed examination to obtain ahigh strength member exceeding 1.5 GPa after hot stamping by increasingthe C content. As a result, it was found that by setting the C contentto 0.25 mass % or more, an ultrahigh strength of 1.5 GPa or more interms of tensile strength can be obtained after hot stamping. On theother hand, it was found that the bendability and toughness deterioratewith ultrahigh-strengthening for a tensile strength of 1.5 GPa or more.In particular, it was found that the bendability of the coated steelmember is significantly lowered in a case where the coated steel sheetsubjected to the Al-based coating as described above is used.

(b) The present inventors examined the effect of structures on thebendability of a coated steel sheet having an Al-based coating such asan Al plating. As a result, it was presumed that since the bendabilityis easily affected by the surface layer of a steel member, and anAl—Fe-based coating present on the surface layer of a coated steelmember is hard, the bendability is deteriorated by the Al—Fe-basedcoating.

(c) The present inventors examined the relationship between thebendability and the surface layer structure of a coated steel memberhaving an Al—Fe-based coating having a high strength of more than 1.5GPa, and worked on improving the bendability. As a result, it was foundthat by reducing the C content unavoidably contained in the Al—Fe-basedcoating, it is possible to soften the Al—Fe-based coating anddramatically improve the bendability. In addition, it was found that Ccontained in the Fe—Al-based coating as described above is diffused fromthe steel sheet during a heat treatment, and by using a coated steelsheet in which the C content is reduced only in the surface layer areaof the steel sheet in advance, it is possible to obtain a coated steelmember having excellent bendability and strength.

(d) The present inventors further worked on improving the toughness ofsteel members in order to prevent premature fracture at the time of acollision. Refinement of the internal structure is effective forimproving toughness, and by refining prior γ grains by precipitates ofNb—Ti—(C,N), it is possible to dramatically improve toughness. However,as a result of the examinations by the present inventors, it was foundthat when Nb—Ti—(C,N) is precipitated inside the coated steel sheet, thediffusion of C in the steel sheet is promoted through the grainboundaries increased by the refinement, the C content in the Fe—Al-basedcoating cannot be sufficiently reduced, and there are cases where thebendability is lowered.

Therefore, the present inventors examined a method for improvingtoughness without lowering bendability. As a result, it was found thatby refining prior γ grains through precipitation of precipitates ofNb—Ti—(C,N) in the steel sheet and simultaneously allowing Si or Ni thatreduces the activity of C to be contained in the upper layer of theAl-based coating or allowing Cr that increases the activity of C to beconcentrated in the lower layer of the Al-based coating, the diffusionof C from the steel sheet to the Fe—Al-based coating can be suppressedin the heat treatment, and as a result, it is possible to simultaneouslyimprove toughness and bendability.

A coated steel member (coated steel member according to the presentembodiment) and a coated steel sheet (coated steel sheet according tothe present embodiment) according to an embodiment of the presentinvention, and methods for producing the same were obtained by the abovefindings. Each requirement will be described in detail below.

(A) Coated Steel Member

As shown in FIG. 1 , a coated steel member 1 according to the presentembodiment has a steel sheet substrate 11 having a predeterminedchemical composition and a coating 12 formed on the surface of the steelsheet substrate 11 and containing Al and Fe.

In addition, the coating (Al—Fe-based coating) 12 has a high Al contentregion 122 having an Al content of 30 mass % or more and a low Alcontent region 121 having an Al content of 3 mass % or more and lessthan 30 mass %, the maximum C content of the high Al content region 122is 25% or less of the C content of the steel sheet substrate 11, themaximum C content of the low Al content region 121 is 40% or less of theC content of the steel sheet substrate 11, and the maximum C content ina range from the interface between the steel sheet substrate 11 and thecoating 12 to a depth of 10 μm on the steel sheet substrate 11 side is80% or less with respect to an average C content of the steel sheetsubstrate 11 excluding the surface layer area.

In the coated steel member 1 according to the present embodiment, asshown in FIG. 1 , the steel sheet substrate 11 side of the coating 12 isthe low Al content region 121, and the surface side is the high Alcontent region 122.

(A1) Chemical Composition of Steel Sheet Substrate

The steel sheet substrate 11 of the coated steel member 1 according tothe present embodiment has a predetermined chemical composition.Specifically, as the chemical composition, by mass %, C: 0.25% to 0.65%,Si: 0.10% to 2.00%, Mn: 0.30% to 3.00%, P: 0.050% or less, S: 0.0100% orless, N: 0.010% or less, Ti: 0.010% to 0.100%, B: 0.0005% to 0.0100%,Nb: 0.02% to 0.10%, Mo: 0% to 1.00%, Cu: 0% to 1.00%, Cr: 0% to 1.00%,Ni: 0% to 1.00%, V: 0% to 1.00%, Ca: 0% to 0.010%, Al: 0% to 1.00%, Sn:0% to 1.00%, W: 0% to 1.00%, Sb: 0% to 1.00%, REM: 0% to 0.30%, and aremainder of Fe and impurities are included.

The reasons for limiting each element are as follows. Here, the chemicalcomposition of the steel sheet substrate refers to the chemicalcomposition of a part of the coated steel member excluding theAl—Fe-based coating of the surface and the surface layer structure ofthe steel sheet substrate. Hereinafter, % regarding the content is mass% unless otherwise specified.

C: 0.25% to 0.65%

C is an element that enhances the hardenability of steel and improvesthe strength of the coated steel member after hardening. However, whenthe C content is less than 0.25%, it becomes difficult to securesufficient strength (more than 1.5 GPa) in the coated steel member afterhardening. Therefore, the C content is set to 0.25% or more. The Ccontent is preferably 0.28% or more.

On the other hand, when the C content exceeds 0.65%, the strength of thecoated steel member after hardening becomes too high, and thedeterioration of toughness and bendability becomes significant.Therefore, the C content is set to 0.65% or less. The C content ispreferably 0.60% or less.

Si: 0.10% to 2.00%

Si is an element that is effective in enhancing the hardenability ofsteel and stably securing the strength after hardening. In order toobtain this effect, 0.10% or more of Si needs to be contained. The Sicontent is preferably 0.35% or more.

On the other hand, when the Si content in the steel exceeds 2.00%, aheating temperature required for austenitic transformation becomessignificantly high during a heat treatment. This may lead to an increasein the cost required for the heat treatment. Furthermore, when the Sicontent exceeds 2.00%, the toughness of a hardened portion deteriorates.Therefore, the Si content is set to 2.00% or less. The Si content ispreferably 1.60% or less.

Mn: 0.30% to 3.00%

Mn is a very effective element for enhancing the hardenability of steeland stably securing the strength after hardening. Mn is an element thatfurther lowers an Ac₃ point and promotes the lowering of a hardeningtreatment temperature. Furthermore, Mn is an element having an effect ofimproving corrosion resistance by being diffused into the Al—Fe-basedcoating. When the Mn content is less than 0.30%, these effects are notsufficient, so that the Mn content is set to 0.30% or more. The Mncontent is more preferably 0.40% or more.

On the other hand, when the Mn content exceeds 3.00%, the above effectsare saturated, and the toughness and bendability of the hardened portiondeteriorate. Therefore, the Mn content is set to 3.00% or less. The Mncontent is preferably 2.80% or less, and more preferably 2.50% or less.

P: 0.050% or Less

P is an element that deteriorates the toughness of the coated steelmember after hardening. In particular, when the P content exceeds0.050%, the deterioration of toughness becomes significant. Therefore,the P content is limited to 0.050% or less. The P content is preferablylimited to 0.005% or less. Since it is preferable that the P content issmall, the P content may be 0%. However, the P content may be set to0.001% or more from the viewpoint of cost.

S: 0.0100% or Less

S is an element that deteriorates the toughness and bendability of thecoated steel member after hardening. In particular, when the S contentexceeds 0.0100%, the deterioration of the toughness and bendabilitybecomes significant. Therefore, the S content is limited to 0.0100% orless. The S content is preferably limited to 0.0050% or less. Since itis preferable that the S content is small, the S content may be 0%.However, the S content may be set to 0.0001% or more from the viewpointof cost.

N: 0.010% or Less

N is an element that deteriorates the toughness of the coated steelmember after hardening. In particular, when the N content exceeds0.010%, coarse nitrides are formed in the steel, and the toughness issignificantly deteriorated. Therefore, the N content is set to 0.010% orless. The lower limit of the N content is not particularly limited andmay be 0%. However, setting the N content to less than 0.0002% leads toan increase in steelmaking cost and is economically undesirable.Therefore, the N content may be set to 0.0002% or more, or 0.0008% ormore.

Ti: 0.010% to 0.100%

Ti is an element having an action of suppressing recrystallization whenthe steel sheet is subjected to a heat treatment by being heated to atemperature of the Ac₃ point or higher, and suppressing grain growth byforming fine carbides, thereby refining austenite grains. Therefore, byincluding Ti, an effect of greatly improving the toughness of the coatedsteel member can be obtained. In addition, Ti is an element thatsuppresses the consumption of B due to the precipitation of BN by beingpreferentially bonded to N in the steel and promotes an effect ofimproving the hardenability by B, which will be described later. Whenthe Ti content is less than 0.010%, the above effects cannot besufficiently obtained. Therefore, the Ti content is set to 0.010% ormore. The Ti content is preferably 0.015% or more.

On the other hand, when the Ti content exceeds 0.100%, the amount of TiCprecipitated increases and C is consumed, resulting in a decrease in thestrength of the coated steel member after hardening. Therefore, the Ticontent is set to 0.100% or less. The Ti content is preferably 0.080% orless.

B: 0.0005% to 0.0100%

B has an action of dramatically improving the hardenability of steeleven in a trace amount and is thus an important element. Furthermore, Bis an element that strengthens the grain boundaries and enhancestoughness by being segregated at the grain boundaries, and is an elementthat suppresses the growth of austenite grains when the steel sheet isheated. When the B content is less than 0.0005%, there are cases wherethe above effects cannot be sufficiently obtained. Therefore, the Bcontent is set to 0.0005% or more. The B content is preferably 0.0010%or more.

On the other hand, when the B content exceeds 0.0100%, a large amount ofcoarse compounds are precipitated, and the toughness of the coated steelmember deteriorates. Therefore, the B content is set to 0.0100% or less.The B content is preferably 0.0080% or less.

Nb: 0.02% to 0.10%

Nb has an action of forming fine carbides and increasing the toughnessof steel due to the refining effect and is thus an important element.When the Nb content is less than 0.02%, there are cases where the aboveeffects cannot be sufficiently obtained. Therefore, the Nb content isset to 0.02% or more. The Nb content is preferably 0.03% or more.

On the other hand, when the Nb content exceeds 0.10%, the carbidesbecome coarse and the toughness of the coated steel member deteriorates.Therefore, the Nb content is set to 0.10% or less. The Nb content ispreferably 0.08% or less.

In order to improve the strength, toughness, bendability, corrosionresistance, and deoxidation of the coated steel member according to thepresent embodiment, in addition to the above elements, one or moreelements selected from Cr, Ni, Cu, Mo, V, Ca, Al, Nb, Sn, W, Sb, and REMdescribed below may be contained. These elements are optional elementsand do not necessarily have to be contained. Therefore, the lower limitthereof is 0%.

Cr: 0% to 1.00%

Cr increases the hardenability of steel, and is an effective element forstably securing the strength of the coated steel member after hardening.Therefore, Cr may be contained. Furthermore, Cr is an element having aneffect of improving corrosion resistance by being diffused into theAl—Fe-based coating. In order to obtain the above effects, the Crcontent is preferably 0.01% or more, more preferably 0.05% or more, andeven more preferably 0.08% or more.

However, when the Cr content exceeds 1.00%, the above effects aresaturated and the cost increases. Moreover, since Cr has an action ofstabilizing iron carbides, when the Cr content exceeds 1.00%, there arecases where coarse iron carbides remain undissolved when the steel sheetis heated, and the toughness of the coated steel member after hardeningdeteriorates. Therefore, the Cr content when contained is set to 1.00%or less. The Cr content is preferably 0.80% or less.

Ni: 0% to 1.00%

Ni increases the hardenability of steel, and is an effective element forstably securing the strength of the coated steel member after hardening.Therefore, Ni may be contained. Furthermore, Ni is an element having aneffect of improving corrosion resistance by being diffused into theAl—Fe-based coating. In order to obtain the above effects, Ni iscontained preferably in an amount of 0.01% or more, and more preferablyin an amount of 0.10% or more.

However, when the Ni content exceeds 1.00%, the above effects aresaturated and the economic efficiency is lowered. Therefore, the Nicontent when contained is set to 1.00% or less.

Cu: 0% to 1.00%

Cu increases the hardenability of steel, and is an effective element forstably securing the strength of the coated steel member after hardening.Therefore, Cu may be contained. Furthermore, Cu is an element having aneffect of improving the corrosion resistance of the steel member. Inorder to obtain the above effects, the Cu content is preferably 0.01% ormore, and more preferably 0.05% or more.

However, when the Cr content exceeds 1.00%, the above effects aresaturated and the cost increases. Therefore, the Cu content whencontained is set to 1.00% or less. The Cu content is preferably 0.80% orless.

Mo: 0% to 1.00%

Mo increases the hardenability of steel, and is an effective element forstably securing the strength of the coated steel member after hardening.Therefore, Mo may be contained. Furthermore, Mo is an element having aneffect of improving corrosion resistance by being diffused into theAl—Fe-based coating. In order to obtain the above effects, the Mocontent is preferably 0.01% or more, and more preferably 0.05% or more.

However, when the Mo content exceeds 1.00%, the above effects aresaturated and the cost increases. Moreover, since Mo has an action ofstabilizing iron carbides, when the Mo content exceeds 1.00%, there arecases where coarse iron carbides remain undissolved when the steel sheetis heated, and the toughness of the coated steel member after hardeningdeteriorates. Therefore, the Mo content when contained is set to 1.00%or less. The Mo content is preferably 0.80% or less.

V: 0% to 1.00%

V is an element that forms fine carbides and increases toughness due tothe refining effect. Therefore, V may be contained. In order to obtainthe above effect, V is contained preferably in an amount of 0.01% ormore, and more preferably in an amount of 0.10% or more.

However, when the V content exceeds 1.00%, the above effects aresaturated and the economic efficiency is lowered. Therefore, the Vcontent when contained is set to 1.00% or less.

Ca: 0% to 0.010%

Ca is an element having an effect of refining inclusions in steel andimproving toughness after hardening. Therefore, Ca may be contained. Ina case where the above effect is obtained, the Ca content is set topreferably 0.001% or more, and more preferably 0.002% or more.

However, when the Ca content exceeds 0.010%, the effect is saturated andthe cost increases. Therefore, in a case where Ca is contained, the Cacontent is set to 0.010% or less. The Ca content is preferably 0.005% orless, and more preferably 0.004% or less.

Al: 0% to 1.00%

Al is an element generally used as a steel deoxidizing agent. Therefore,Al may be contained. In order to obtain the above effect, Al ispreferably contained in an amount of 0.01% or more.

However, when the Al content exceeds 1.00%, the above effect issaturated and the economic efficiency is lowered. Therefore, the Alcontent when contained is set to 1.00% or less.

Sn: 0% to 1.00%

Sn is an element that improves corrosion resistance in a corrosiveenvironment. Therefore, Sn may be contained. In order to obtain theabove effect, Sn is preferably contained in an amount of 0.01% or more.

However, when the Sn content exceeds 1.00%, the grain boundary strengthdecreases, and the toughness of the coated steel member after hardeningdeteriorates. Therefore, the Sn content when contained is set to 1.00%or less.

W: 0% to 1.00%

W is an element that makes it possible to increase the hardenability ofsteel and stably secure the strength of the coated steel member afterhardening. Therefore, W may be contained. Furthermore, W is an elementthat improves corrosion resistance in a corrosive environment. In orderto obtain the above effects, W is preferably contained in an amount of0.01% or more.

However, when the W content exceeds 1.00%, the above effects aresaturated and the economic efficiency is lowered. Therefore, the Wcontent when contained is set to 1.00% or less.

Sb: 0% to 1.00%

Sb is an element that improves corrosion resistance in a corrosiveenvironment. Therefore, Sb may be contained. In order to obtain theabove effect, the Sb content is preferably set to 0.01% or more.

However, when the Sb content exceeds 1.00%, the grain boundary strengthdecreases, and the toughness of the coated steel member after hardeningdeteriorates. Therefore, the Sb content when contained is set to 1.00%or less.

REM: 0% to 0.30%

Like Ca, REM is an element having an effect of refining inclusions insteel and improving the toughness of the coated steel member afterhardening. Therefore, REM may be contained. In a case where the aboveeffect is desired, the REM content is set to preferably 0.01% or more,and more preferably 0.02% or more.

However, when the REM content exceeds 0.30%, the effect is saturated andthe cost increases. Therefore, the REM content when contained is set to0.30% or less. The REM content is preferably 0.20% or less.

Here, REM refers to a total of 17 elements of Sc, Y, and lanthanoidssuch as La and Nd, and the REM content means the total amount of theseelements. REM is added to molten steel using, for example, an Fe—Si-REMalloy, and this alloy contains, for example, La, Nd, Ce, Pr.

In the chemical composition of the coated steel member of the presentembodiment, the remainder other than the elements described aboveconsists of Fe and impurities.

Here, the “impurities” are elements that are incorporated due to variousfactors including raw materials such as ore and scrap and the productionprocess when the steel sheet is industrially produced, and areacceptable in a range without adversely affecting the properties of thecoated steel member according to the present embodiment.

The chemical composition of the steel sheet substrate can be obtained bythe following method.

The chemical composition of the steel sheet substrate is obtained bycutting out an analysis sample from the steel sheet substrate andperforming elemental analysis such as inductively coupled plasma (ICP)atomic emission spectrometry. The analysis sample is collected so as toobtain an average chemical composition of the overall sheet thickness ofthe steel sheet substrate, as described in JIS G 0417. Specifically, theanalysis sample is collected from a thickness ¼ position in the sheetthickness direction from the surface of the steel sheet substrate,avoiding width direction end portions of the coated steel member.

The average C content and average Cr content excluding the surface layerarea of the steel sheet substrate are the values obtained from the aboveICP atomic emission spectrometry.

(A2) Coating

The coated steel member 1 according to the present embodiment has thecoating 12 containing Al and Fe (hereinafter, Al—Fe-based coating) onthe surface of the steel sheet substrate 11 described above. In thepresent embodiment, the Al—Fe-based coating 12 is a coating primarilycontaining Al and Fe, and preferably contains Al and Fe in a totalamount of 70% or more. In addition, the Al—Fe-based coating 12 is alsoreferred to as a film, an alloy plating layer, or an intermetalliccompound layer. The Al—Fe-based coating 12 further contains Si, Mg, Ca,Ni, Cu, Mn, Cr, Mo, Sn, Sr, C, and the like, and the remainder may beimpurities.

The coating (Al—Fe-based coating 12) included in the coated steel member1 according to the present embodiment has the high Al content region 122having an Al content of 30 mass % or more and the low Al content region121 having an Al content of 3 mass % or more and less than 30 mass %. Inaddition, the maximum C content of the high Al content region 122 is 25%or less of the C content of the steel sheet substrate 11, and themaximum C content of the low Al content region 121 is 40% or less of theC content of the steel sheet substrate 11.

The kind of coating is not limited. For example, the coating is acoating formed by hot-dip plating, electro plating, thermal spraying, orthe like.

The thickness of the Al—Fe-based coating is preferably 5 to 50 μm.

By controlling the Al—Fe-based coating 12 as described above, thebendability of the coated steel member 1 is improved. When the maximum Ccontent of the high Al content region 122 exceeds 25% of the C content(average C content in the steel sheet substrate) contained in the steelsheet substrate 11, and/or the maximum C content of the low Al contentregion 121 exceeds 40% of the C content in the steel sheet substrate 11,there are cases where the bendability is not sufficient and energycannot be sufficiently absorbed at the time of a collision. The lowerlimit of the C content in the high Al content region 122 or the Ccontent in the low Al content region 121 is not particularly specified,but may be about 0.1% of the C content in the steel sheet substrate 11.

The maximum C content in the Al—Fe-based coating 12 can be obtained asfollows.

The maximum C contents of the high Al content region 122 and the low Alcontent region 121 are obtained by performing glow discharge emissionanalysis (GDS) on the coating in the thickness direction from thesurface of the coated steel member 1. Specifically, glow dischargeemission analysis (GDS) is performed at an approximately ¼ position ofwidth (short direction) from the width direction end portion of thecoated steel member 1 in the sheet thickness direction from the surfaceof the coated steel member 1 to obtain the maximum C content in a rangewhere the Al content is 30 mass % or more, and the maximum C content ina range where the Al content is 3 mass % or more and less than 30 mass%. This measurement is performed a total of five times, and the averagevalue of the maximum C contents obtained in each measurement isdetermined to be the maximum C content of the high Al content region 122and the maximum C content in the low Al content region 121.

In the present embodiment, the Al—Fe-based coating 12 is a region wherethe Fe content is less than 95% when measured by GDS from the surface ofthe coated steel member.

The maximum Cr content in the high Al content region is preferably 80%or more of the Cr content of the steel sheet substrate (average Crcontent in the steel sheet substrate). In this case, an effect ofimproving the corrosion resistance of the coated steel member can beobtained.

The maximum Cr content in the high Al content region 122 is determinedby performing glow discharge emission analysis (GDS) on the coating inthe thickness direction from the surface of the coated steel member inthe same manner as the maximum C content.

(A3) Surface Layer Structure of Steel Sheet Substrate

In the coated steel member 1 according to the present embodiment, themaximum C content in a range from the surface of the steel sheetsubstrate 11 (the interface with the steel sheet substrate 11 and thecoating 12) to a depth of 10 μm (a range from the interface to 10 μm onthe steel sheet substrate side, so-called surface layer structure of thesteel sheet substrate) is 80% or less of the C content (average Ccontent) of the steel sheet substrate 11.

The maximum C content in the range from the interface between the steelsheet substrate 11 and the coating 12 to the depth of 10 μm on the steelsheet substrate 11 side is obtained by performing glow dischargeemission analysis (GDS) in the sheet thickness direction from thesurface of the coated steel member 1. Specifically, glow dischargeemission analysis (GDS) is performed at an approximately ¼ position ofwidth (short direction) from the width direction end portion of thecoated steel member 1 in the sheet thickness direction from the surfaceof the coated steel member 1 to determine a region where the Fe contentis less than 95 mass % to be the Al—Fe-based coating 12, and determine aregion where the Fe content is 95 mass % or more to be the steel sheetsubstrate 11. In the steel sheet substrate 11, the maximum C content inthe range from the interface between the Al—Fe-based coating 12 and thesteel sheet substrate 11 to a depth of 10 μm is obtained. Thismeasurement is performed five times, and the average value of themaximum C contents obtained in each measurement is determined to be themaximum C content in the range from the surface of the steel sheetsubstrate to the depth of 10 μm.

(A4) Internal Structure of Steel Sheet Substrate

The internal structure of the steel sheet substrate 11 of the coatedsteel member 1 according to the present embodiment is a structureprimarily containing martensite and bainite, which have high strength.In terms of area fraction, the sum of martensite and bainite ispreferably 90% or more, the sum of the martensite and bainite is morepreferably 90% or more, and martensite occupies 70% or more in terms ofarea fraction. More preferably, martensite occupies 80% or more. Theinternal structure of the steel sheet substrate 11 is the structure of aregion excluding the surface layer structure (10 μm from the surface) ofthe steel sheet substrate 11 described above.

The internal structure of the steel sheet substrate 11 may containresidual austenite, bainite, ferrite, or pearlite as the remainder otherthan martensite and bainite. Martensite includes not only so-calledfresh martensite but also tempered martensite and auto-temperedmartensite. The auto-tempered martensite is tempered martensitegenerated during cooling at the time of hardening without a heattreatment for tempering, and is generated by in-situ tempering ofmartensite generated due to self-heating associated with martensitictransformation.

The internal structure of the steel sheet substrate can be determined bythe following method.

The total area fraction of martensite (including tempered martensite andauto-tempered martensite) and bainite is obtained by an X-raydiffraction method. Specifically, a measurement sample is cut out from a¼ position of width from the width direction end portion of the steelmember and used as a sample for X-ray diffraction. This sample ischemically polished from the surface of the steel sheet substrate to athickness ¼ depth using hydrofluoric acid and hydrogen peroxidesolution. The polished sample is subjected to X-ray diffraction in a 20range of 45° to 105° using a Co tube bulb as a measurement condition.The diffracted X-ray intensities of a body-centered cubic lattice(martensite and bainite) and a face-centered cubic lattice (residualaustenite) included in the steel member are measured, the total volumefraction of martensite and bainite and the volume fraction of residualaustenite are obtained from the area ratio of the diffraction curve.Since the steel member of the present embodiment has an isotropymetallographic structure, the values of the volume fractions can bedirectly replaced into area fractions. Accordingly, the total areafraction of martensite and bainite is obtained. Ferrite or pearlite inthe body-centered cubic lattice in a case of being mixed can be easilyidentified by an optical microscope or a scanning microscope describedlater.

In a case where the area fractions of martensite and bainite areseparately measured, the measurement is performed by a transmissionelectron microscope (TEM) and an electron beam diffractometer attachedto the TEM. A measurement sample is cut out from a ¼ position of thewidth of the steel member or a ¼ position of the thickness of the steelsheet substrate to be used as a thin film sample for TEM observation. Asthe thin film sample, a sample cut out from a cross section in thedirection perpendicular to a rolling direction is used. The range of TEMobservation is set to a range of 400 μm² in terms of area. The electronbeam diffraction pattern of the thin film sample makes it possible todistinguish between martensite and bainite, which are body-centeredcubic lattices, and residual austenite, which is a face-centered cubiclattice. Iron carbides (Fe₃C) in martensite and bainite are found by thediffraction pattern, and the precipitation morphology thereof isobserved to separate martensite and bainite from each other and measurethe microstructural fractions thereof. Specifically, regarding theprecipitation morphology, precipitation with three orientations isdetermined to be martensite, and precipitation limited to oneorientation is determined to be bainite. Carbides are observed todistinguish between martensite and bainite, but in the presentembodiment, carbides are not included in the volume fraction of thestructure.

Ferrite or pearlite that may be present as the remainder inmicrostructure can be easily confirmed with an optical microscope or ascanning electron microscope. Specifically, a measurement sample is cutout from the ¼ position of the width of the steel member and the ¼position of the thickness of the steel sheet substrate to be used as asample for observation. As the sample, a sample cut out from a crosssection in the direction perpendicular to the rolling direction is used.The observation range of the microscope is set to a range of 40,000 μm²in terms of area. The cut sample is mechanically polished and thenmirror-finished. Next, etching is performed with a nital etchingsolution to reveal ferrite and pearlite, and this is observed with themicroscope to confirm the presence of ferrite or pearlite. A structurein which ferrite and cementite are alternately arranged in layers isdetermined to be pearlite, and a structure in which cementite isprecipitated in particles is determined to be bainite.

(A5) Properties of Coated Steel Member

In the coated steel member 1 according to the present embodiment, thesteel sheet substrate 11 and the coating 12 are controlled as describedabove, so that the Al—Fe-based coating 12 is softened and thebendability at the time of a collision is improved. In addition, thecoated steel member 1 according to the present embodiment has not onlyexcellent bendability but also a tensile strength as high as more than1.5 GPa, and also has excellent toughness.

In the present embodiment, the bendability is evaluated by a collisiontest or a bending test of the coated steel member. For example, abending test piece is cut out from the coated steel member, a bendingtest is performed in accordance with VDA standard 238-100, and thebendability is evaluated by the bending angle at the maximum load.

In the coated steel member 1 according to the present embodiment, it ispossible to obtain excellent bendability with a bending angle of 60degrees or more at the maximum load in a case where the tensile strengthis more than 1.5 GPa to 2.1 GPa and with a bending angle of 50 degreesor more in a case where the tensile strength is more than 2.1 GPa.

In addition, in the present embodiment, the toughness is evaluated by acollision test or a Charpy impact test of the coated steel member. Forexample, a V-notch Charpy impact test piece having a size of sheetthickness×10 mm×55 mm is cut out from the steel member, and a Charpyimpact test is performed in accordance with JIS Z 2242:2018 to evaluatethe toughness by an impact value at −40° C.

In the coated steel member according to the present embodiment, it ispossible to obtain excellent toughness with an impact value of 35 J/cm²or more in a case where the tensile strength is more than 1.5 GPa to 2.1GPa and with an impact value of 20 J/cm² or more in a case where thetensile strength is more than 2.1 GPa.

The shape of the coated steel member is not particularly limited. Thatis, the shape thereof may be a flat sheet or a formed body. A hot-formedcoated steel member is often a formed body, and in the presentembodiment, a case of a formed body and a case of a flat sheet arecollectively referred to as a “coated steel member”.

(B) Coated Steel Sheet

Next, a coated steel sheet 2 according to the present embodiment will bedescribed. The coated steel sheet 2 according to the present embodimentis suitable as a material for the coated steel member 1 according to theabove-described embodiment.

As shown in FIG. 2 , the coated steel sheet 2 according to the presentembodiment has a steel sheet 21 having a predetermined chemicalcomposition and a coating (Al-based coating) 22 formed on the surface ofthe steel sheet 21 and containing Al. In addition, the Al-based coatingis provided on the steel sheet side, and includes a lower layer 221containing 3 mass % or more and less than 70 mass % of Al, and an upperlayer 222 containing 70 mass % or more and 95 mass % or less of Al.Furthermore, the Al-based coating 22 satisfies any of (i) and (ii)below. Preferably, both (i) and (ii) are satisfied.

(i) The lower layer 221 contains Cr in an amount of 1.2 times or morethe Cr content in the steel sheet 21 by mass %. (ii) The upper layer 222contains Si and Ni in a total amount of 5.0 mass % or more and 30.0 mass% or less.

Furthermore, in the coated steel sheet 2 according to the presentembodiment, in the steel sheet 21, the maximum C content in a range fromthe interface between the steel sheet 21 and the Al-based coating 22 toa depth of 20 μm on the steel sheet 21 side (surface layer structure ofthe steel sheet 21) is 80% or less of the average C content excludingthe surface layer area of the steel sheet 21.

(B1) Chemical Composition of Steel Sheet

The chemical composition of the steel sheet 21 included in the coatedsteel sheet 2 is the same as the chemical composition of the steel sheetsubstrate 11 in the coated steel member 1 described above, and thereason for its limitation is also the same. Here, the chemicalcomposition of the steel sheet 21 refers to the chemical composition ofa part of the coated steel sheet 2 excluding the Al-based coating 22 ofthe surface and the surface layer structure of the steel sheet 21, andis obtained by performing elemental analysis such as inductively coupledplasma (ICP) atomic emission spectrometry at a thickness ¼ position fromthe surface in the sheet thickness direction of the steel sheet. Theaverage C content and average Cr content excluding the surface layerarea of the steel sheet are values obtained from the above ICP atomicemission spectrometry.

(B2) Coating

The coated steel sheet 2 according to the present embodiment has theAl-containing coating (hereinafter, Al-based coating) 22 on the surfaceof the steel sheet 21. The Al-based coating 22 has the lower layer 221containing 3 mass % or more and less than 70 mass % of Al on the steelsheet 21 side of the coated steel sheet 2, and the upper layer 222containing 70 mass % or more and 95 mass % or less of Al on the surfaceside of the coated steel sheet 2. The Al-based coating is a coatingprimarily containing Al. For example, to an Al-based coating containing3% or more of Al, as additive elements, Mg, Ti, Zn, Sb, Sn, Cu, Co, In,Bi, Ca, Sr, mischmetal, and the like may be added.

The upper layer 222 preferably contains Si and Ni in a total amount of5.0 mass % or more and 30.0 mass % or less. Si and Ni contained in theupper layer 222 of the Al-based coating 22 are elements that lower theactivity of C, and when the maximum total amount of Si and Ni in theupper layer 222 is 5.0 mass % or more, an effect of suppressing thediffusion of C from the steel sheet to the Al—Fe-based coating(particularly to the surface side of the Al—Fe-based coating) can beobtained in a heat treatment described later. When the maximum totalamount of Si and Ni is less than 5.0 mass %, there are cases where the Ccontent in the Al—Fe-based coating 12 when a heat treatment is performedto obtain the coated steel member 1 increases. In this case, thebendability is not sufficient, and energy cannot be sufficientlyabsorbed at the time of a collision. Therefore, the total amount of Siand Ni in the upper layer 222 is preferably set to 5.0 mass % or more.

Cr is an element that increases the activity of C. Therefore, when Cr iscontained in the lower layer 221 present on the steel sheet 21 side ofthe Al-based coating 22, there is an effect of suppressing the diffusionof C into the Al—Fe-based coating in the heat treatment described later.Therefore, the lower layer 221 preferably contains Cr.

Furthermore, by including Cr in the lower layer 221, Cr can also becontained in the high Al content region 122 when the coated steel member1 is obtained.

In a case where the above effect is obtained, it is preferable that thesteel sheet 21 contains 0.05% or more of Cr, and the maximum value ofthe Cr content in the lower layer 221 is 1.2 times or more (120% ormore) the Cr content (average Cr content) in the steel sheet 21. Morepreferably, the steel sheet 21 contains 0.08% or more of Cr, and themaximum value of the Cr content in the lower layer 221 is 1.2 times ormore (120% or more) the Cr content in the steel sheet 21.

When the maximum value of the Cr content in the lower layer 221 is lessthan 120% of the Cr content of the steel sheet 21, the effect ofsuppressing the diffusion of C into the Al—Fe-based coating 12 is small.The lower limit of the Cr content in the lower layer 221 is notparticularly specified, but may be about 0.01% in a case where the steelsheet 21 contains Cr.

In the coated steel sheet 2 according to the present embodiment, inorder to suppress the diffusion of C into the Al—Fe-based coating 12after the heat treatment, the upper layer 222 needs to contain Si and Niin a total amount of 5.0 mass % or more and 30.0 mass % or less, or thelower layer 221 needs to contain Cr in an amount of 1.2 times or morethe Cr content in the steel sheet by mass %. The diffusion of C can besuppressed as long as at least one thereof is satisfied. In a case ofobtaining a higher effect, it is preferable to satisfy both.

The maximum values of the total amount of Si and Ni of the upper layer222 and the Cr content of the lower layer 221 in the Al-based coating 22are obtained by performing glow discharge emission analysis (GDS) in thesheet thickness direction from the surface of the coated steel sheet 2as follows.

When obtaining the maximum values of the total amount of Si and Ni ofthe upper layer 222 and the Cr content of the lower layer 221, a regionwhere the Al content is 3 mass % or more and the Fe content is less than95 mass % is determined to be the Al-based coating 22, and a regionwhere the Fe content is 95 mass % or more is determined to be the steelsheet 21. Furthermore, in the Al-based coating 22, a region where the Alcontent is 70 mass % or more is determined to be the upper layer 222,and a region where the Al content is less than 70% is determined to belower layer 221.

Specifically, glow discharge emission analysis (GDS) is performed at anapproximately ¼ position of width (short direction) from the widthdirection end portion of the coated steel member in the sheet thicknessdirection from the surface of the coated steel sheet to obtain the Sicontent and the Ni content in the upper layer, and the sum of the Sicontent and the Ni content at a position where the total content is thelargest is used as the total amount of Si and Ni of the upper layer inthe upper layer.

Furthermore, glow discharge emission analysis (GDS) is performed at anapproximately ¼ position of width (short direction) from the widthdirection end portion of the coated steel member in the sheet thicknessdirection from the surface of the coated steel sheet to measure themaximum value of the Cr content in the lower layer.

This measurement is performed a total of five times, and the valuesobtained in each measurement are averaged to obtain the maximum valuesof the total amount of Si and Ni of the upper layer and the Cr contentof the lower layer.

(B3) Surface Layer Structure of Steel Sheet

In the coated steel sheet 2 according to the present embodiment, themaximum C content in the range (surface layer structure of the steelsheet 21) from the surface of the steel sheet 21 (the interface betweenthe steel sheet 21 and the coating 22) to a depth of 20 μm is 80% orless of the C content (average C content) of the steel sheet 21.

When the maximum C content in the surface layer structure of the steelsheet 21 exceeds 80% of the C content of the steel sheet 21, the Ccontent in the Al—Fe-based coating 12 of the coated steel member 1 afterthe heat treatment becomes high. In this case, there are cases where thebendability is not sufficient, and energy is not sufficiently absorbedat the time of a collision. The lower limit of the maximum C content inthe range from the surface of the steel sheet 21 to the depth of 20 μmis not particularly specified, but may be about 0.01% in total.

The C content in the steel sheet 21 can be obtained as follows.

The maximum C content in the range from the surface of the steel sheet21 to the depth of 20 μm is obtained by performing glow dischargeemission analysis (GDS) in the sheet thickness direction from thesurface of the coated steel sheet 2. Specifically, glow dischargeemission analysis (GDS) is performed at an approximately ¼ position ofwidth (short direction) from the width direction end portion of thecoated steel sheet 2 in the sheet thickness direction from the surfaceof the coated steel sheet 2 to determine a region where the Fe contentis less than 95 mass % to be the Al-based coating 22, and determine aregion where the Fe content is 95 mass % or more to be the steel sheet21. In the steel sheet 21, the maximum C content in the range from theinterface between the Al-based coating 22 and the steel sheet substrate21 to a depth of 20 μm is obtained. This measurement is performed fivetimes, and the average value of the maximum C contents obtained in eachmeasurement is determined to be the maximum C content in the range fromthe surface of the steel sheet to the depth of 20 μm.

(B4) Internal Structure of Steel Sheet

The internal structure of the steel sheet 21 included in the coatedsteel sheet 2 according to the present embodiment is not limited, but isoften ferrite or pearlite. In the conditions of a production methoddescribed later, bainite, martensite, and residual austenite may becontained. The martensite includes not only so-called fresh martensitebut also tempered martensite and auto-tempered martensite. Theauto-tempered martensite is tempered martensite generated during coolingat the time of hardening without a heat treatment for tempering, and isgenerated by in-situ tempering of martensite generated due to heatingassociated with martensitic transformation. The internal structure ofthe steel sheet is a structure excluding the above-mentioned surfacelayer structure.

Next, a method for producing the coated steel sheet 2 and the coatedsteel member 1 will be described.

(C) Method for Producing Coated Steel Sheet

The coated steel sheet 2 according to the present embodiment can bemanufactured by using a production method I or a production method IIincluding the following steps. The production method I and theproduction method II may be performed in combination (for example,performed so as to include steps (i)+(ii)+(iii)+(iv)+(v)+(vi)′+(vii)′).

Production Method I

(i) A slab preparation step of melting and casting a steel having theabove-mentioned chemical composition.

(ii) A hot rolling step of hot-rolling the obtained slab to obtain ahot-rolled steel sheet.

(iii) A coiling step of coiling the hot-rolled steel sheet.

(iv) A hot-rolled sheet annealing step of annealing the hot-rolled steelsheet after the coiling step in an atmosphere containing 80% or more ofnitrogen at 450° C. to 800° C. for 5 hours or longer.

(v) A cold rolling step of descaling the hot-rolled steel sheet afterthe hot-rolled sheet annealing step as necessary, and cold-rolling thehot-rolled steel sheet to obtain a cold-rolled steel sheet.

(vi) An annealing step of annealing the hot-rolled steel sheet or thecold-rolled steel sheet to obtain an annealed steel sheet as necessary.

(vii) A coating step of forming an Al-based coating on the hot-rolledsteel sheet, the cold-rolled steel sheet, or the annealed steel sheet toobtain a coated steel sheet.

Production Method II

(i) A slab preparation step of melting and casting a steel having theabove-mentioned chemical composition to obtain a slab.

(ii) A hot rolling step of hot-rolling the obtained slab to obtain ahot-rolled steel sheet.

(iii) A coiling step of coiling the hot-rolled steel sheet.

(iv)′ A hot-rolled sheet annealing step of performing hot-rolled sheetannealing on the hot-rolled steel sheet as necessary.

(v) A cold rolling step of descaling the hot-rolled steel sheet asnecessary, and cold-rolling the hot-rolled steel sheet to obtain acold-rolled steel sheet.

(vi)′ An annealing step of annealing the hot-rolled steel sheet or thecold-rolled steel sheet to obtain an annealed steel sheet in anatmosphere having a dew point of 1° C. or higher and in a temperaturerange of 700° C. to 950° C.

(vii)′ A coating step of forming an Al-based coating on the surface ofthe annealed steel sheet to obtain a coated steel sheet by immersing theannealed steel sheet in a plating bath containing Si and Ni in a totalamount of 7.0 to 30.0 mass % and a remainder of Al and impurities.

Hereinafter, each step will be described.

Slab Preparation Step

In the slab preparation step, a steel having the above-mentionedchemical composition is melted and cast to produce a slab to besubjected to hot rolling. For example, a slab produced by melting moltensteel having the above chemical composition using a converter or anelectric furnace and performing a continuous casting method thereon canbe used. Instead of the continuous casting method, an ingot-makingmethod, a thin slab casting method, or the like may be adopted.

Hot Rolling Step

In the hot rolling step, the slab is heated, subjected to rough rolling,then subjected to descaling as necessary, and finally subjected tofinish rolling. The hot rolling conditions are not limited.

Coiling Step

In the coiling step, for example, the hot-rolled steel sheet after thehot rolling is coiled in a temperature range of 800° C. or lower. Whenthe coiling temperature exceeds 800° C., the hot-rolled steel sheet iscoiled while transformation hardly progresses and the transformationprogresses in the coil, so that there are cases where the coil shape isdefective.

Hot-Rolled Sheet Annealing Step

In a case where the Cr content in the region (lower layer) containing 3mass % or more and less than 70 mass % of Al is set to 120% or more ofthe Cr content in the steel sheet, in the annealing step of thehot-rolled steel sheet, annealing is performed in an atmospherecontaining 80 vol % or more of nitrogen at 450° C. to 800° C. for 5hours or longer.

Cr is concentrated on the surface layer of the steel sheet by annealingin a non-oxidizing atmosphere. Specifically, by setting the atmosphereto an atmosphere containing 80% or more of a neutral or inert gas suchas nitrogen, and performing annealing in a temperature range of 450° C.to 800° C. for 5 hours or longer, the Cr content in the lower layer ofthe Al-based coating can be 1.2 times (120%) or more the Cr content inthe steel sheet. Nitrogen is desirable as the neutral or inert gas fromthe viewpoint of cost, but argon, helium, or the like may be used. Whenthe annealing atmosphere contains the neutral or inert gas in an amountof less than 80% and/or the retention time is shorter than 5 hours,there are cases where the Cr content in the lower layer of the Al-basedcoating is less than 120% of the Cr content in the steel sheet. Theupper limit of the annealing time is not particularly set, but 48 hoursor longer is not preferable because the cost may become excessive. Whenthe annealing temperature is lower than 450° C., there are cases wherethe Cr content in the lower layer is less than 120% of the Cr content inthe steel sheet, which is not preferable. When the annealing temperatureexceeds 800° C., the cost of the heat treatment becomes excessive, whichis not preferable.

In addition, by performing hot-rolled sheet annealing under the aboveconditions, the maximum C content in the range from the surface of thesteel sheet to the depth of 20 μm can be 80% or less of the C content(average content) of the steel sheet.

In a case of performing this hot-rolled sheet annealing step, thehot-rolled sheet annealing step needs to be performed on the hot-rolledsteel sheet in the black-skinned state in which pickling or the like isnot performed (a state where iron scale consisting of FeO, Fe₂O₃, Fe₃O₄,and the like is formed on the surface). By performing annealing on theblack-skinned hot-rolled steel sheet under the above conditions, a COdesorption reaction on the surface of the steel sheet due to thereduction of iron scale, that is, decarburization progresses, and the Crcontent and C content can be controlled to desirable ranges.

Cold Rolling Step

In the cold rolling step, the hot-rolled steel sheet after thehot-rolled sheet annealing step (in a case where the hot-rolled sheetannealing step is not performed, the hot-rolled steel sheet after thecoiling step) is subjected to descaling and cold-rolled to obtain acold-rolled steel sheet. Descaling and cold rolling do not necessarilyhave to be performed. However, in a case where cold rolling isperformed, the cumulative rolling reduction rolling reduction in thecold rolling is preferably set to 30% or more from the viewpoint ofsecuring good flatness. On the other hand, in order to prevent the loadfrom becoming excessive, the cumulative rolling reduction rollingreduction in the cold rolling is preferably set to 80% or less.

The descaling method is not particularly limited, but pickling ispreferable. In a case where pickling is performed, it is preferable toremove only the iron scale by pickling with hydrochloric acid orsulfuric acid as for the conditions. By performing pickling under theseconditions, a Cr-enriched layer is likely to remain.

Annealing Step

In order to set the maximum C content in the range from the surface ofthe steel sheet to the depth of 20 μm to 80% or less of the C content ofthe steel sheet, the hot-rolled steel sheet or the cold-rolled steelsheet is annealed in a wet hydrogen atmosphere having a dew point of 1°C. or higher and in a temperature range of 700° C. to 950° C. Forexample, in a two-stage heating furnace of a direct fired burner and aradiant tube, the steel sheet is heated to 560° C. to 650° C. with anair-fuel ratio of 0.9 to 1.2, and heated to 700° C. to 950° C. in anatmosphere having a hydrogen concentration of 1% to 13% and a dew pointof 1° C. or higher. A preferable dew point is 3° C. or higher, and amore preferable dew point is 5° C. or higher. It is desirable to holdthe steel sheet in a high dew point atmosphere containing hydrogen at700° C. or higher for 30 seconds or longer. By performing annealingunder the above conditions, a CO desorption reaction of the steel sheetsurface due to the reduction of moisture in the atmosphere progresses,and the maximum C content in the range from the surface of the steelsheet to the depth of 20 μm can be 80% or less of the C content of thesteel sheet. When the dew point exceeds +20° C., an internal oxidationof easily oxidizable elements such as Si and Mn of the steel sheetprogresses excessively, and there are cases where the weldability andthe quality of the surface layer of the steel sheet deteriorate. Inaddition, the cost becomes excessive. Therefore, the dew point ispreferably +20° C. or lower.

However, since the same effect can be obtained by the above-mentionedhot-rolled sheet annealing, this annealing step may be omitted in thecase where the hot-rolled sheet annealing is performed or may beperformed under conditions other than the above conditions.

Coating Step

In the coating step, an Al-based coating is applied to form an Al-basedcoating on the surface of the steel sheet to obtain a coated steelsheet. A method for forming the Al-based coating is not particularlylimited, and a hot-dip plating method, an electro plating method, avacuum vapor deposition method, a cladding method, a thermal sprayingmethod, and the like can be used. The hot-dip plating method is the mostpopular in the industry.

In a case where hot-dip plating is performed, Fe is mixed in the platingbath as an impurity in addition to Al in many cases. Furthermore, inaddition to the above-mentioned elements, Si, Ni, Mg, Ti, Zn, Sb, Sn,Cu, Co, In, Bi, Ca, mischmetal, and the like may be contained in theplating bath as long as 70% or more of Al is contained.

In a case of performing plating, plating may be performed after coolingthe annealed steel sheet after the annealing step to room temperature,or dip plating may be performed by cooling to 650° C. to 750° C. afterthe annealing without temporarily cooling to room temperature.

In the method for producing a coated steel sheet according to thepresent embodiment, in a case where the total content of Si and Nicontained in the region (upper layer) containing 70 mass % or more of Alis set to 5.0 to 30.0 mass %, in the case of hot-dip plating, theAl-based coating is applied using the plating bath containing Si and Niin a total amount of 7.0 to 30.0 mass %. For example, the Al-basedcoating is applied by immersing the annealed steel sheet in a platingbath containing Si and Ni in a total amount of 7.0 to 30.0 mass % ormore and the remainder of Al and impurities. Even in a case where Si orNi is contained in a total amount of 7.0% or more, Mg, Ti, Zn, Sb, Sn,Cu, Co, In, Bi, Ca, mischmetal, and the like may be contained asdescribed above.

However, in a case where the above-mentioned hot-rolled sheet annealingis performed, Cr is concentrated in the lower layer of the coating andthe diffusion of C can be suppressed. Therefore, the amount of Si and Niof the upper layer of the coating does not have to be set to 5.0 mass %or more. Therefore, in a case where the hot-rolled sheet annealing isperformed, in this coating step, the plating bath may not contain Si andNi in a total amount of 7.0 to 30.0 mass %.

Pretreatments and post-treatments of the Al-based coating are notparticularly limited, and precoating, solvent coating, an alloyingtreatment, and the like are possible.

(D) Method for Producing Coated Steel Member

Next, a method for producing the coated steel member according to thepresent embodiment will be described.

By subjecting the coated steel sheet produced as described above to aheat treatment described later, it is possible to obtain the coatedsteel member according to the present embodiment having the high Alcontent region and the low Al content region, in which the maximum Ccontent of the high Al content region is 25% or less of the C content ofthe steel sheet substrate, the maximum C content of the low Al contentregion is 40% or less of the C content of the steel sheet substrate, andthe maximum C content in the range from the interface between the steelsheet substrate and the coating to the depth of 10 μm in the steel sheetsubstrate is 80% or less of the C content of the steel sheet substrate.

That is, by performing the heat treatment on the coated steel sheet tothe present embodiment, it is possible to obtain the coated steel memberaccording to the present embodiment. In a case where the heat treatmentis performed using the coated steel sheet produced by performing thehot-rolled sheet annealing, it is possible to obtain a coated steelmember in which the maximum Cr content in the high Al content region is80% or more of the Cr content of the steel sheet substrate.

For example, the heat treatment conditions are conditions under whichthe coated steel sheet obtained by the above method is heated to an Ac₃point to (Ac₃ point+300)° C. at a temperature rising rate of 1.0 to1000° C./s and is cooled to an Ms point or lower at an upper criticalcooling rate or more.

When the temperature rising rate is less than 1.0° C./s, theproductivity of the heat treatment decreases, which is not preferable.On the other hand, when the temperature rising rate exceeds 1000° C./s,a duplex grain structure is formed and the toughness decreases, which isnot preferable.

When the heat treatment temperature is lower than the Ac₃ point, ferriteremains after cooling and the strength and toughness are insufficient,which is not preferable. On the other hand, it is preferable that whenthe heat treatment temperature exceeds the Ac₃ point+300° C., themaximum C content of the high Al content region becomes more than 25% ofthe C content of the steel sheet substrate, the maximum C content of thelow Al content region becomes more than 40% of the C content of thesteel sheet substrate, and the maximum C content in the range from theinterface between the steel sheet substrate and the coating to the depthof 10 μm in the steel sheet substrate becomes more than 80% of the Ccontent of the steel sheet substrate. In addition, the structure becomescoarse and the toughness decreases, which is not preferable.

The upper critical cooling rate is the minimum cooling rate at whichaustenite is supercooled to generate martensite without causingprecipitation of ferrite and pearlite in the structure. When cooling isperformed at lower than the upper critical cooling rate, ferrite andpearlite are generated, resulting in insufficient strength.

At the time of heating, holding for 1 to 300 seconds may be performed.In addition, after cooling, a tempering treatment at about 100° C. to600° C. may be performed in order to adjust the strength of the steelmember.

The Ac₃ point, the Ms point, and the upper critical cooling rate aremeasured by the following method.

A strip-shaped test piece having a width of 30 mm and a length of 200 mmis cut out from a steel sheet having the same chemical composition asthe coated steel member according to the present embodiment, and thetest piece is heated to 1000° C. at a temperature rising rate of 10°C./s in a nitrogen atmosphere, held at the temperature for 5 minutes,and then cooled to room temperature at various cooling rates. Thecooling rate is set at intervals of 10° C./s from 1° C./s to 100° C./s.By measuring the change in thermal expansion of the test piece duringheating and cooling at that time, the Ac₃ point and the Ms point aremeasured.

Among the test pieces cooled at the above cooling rates, the minimumcooling rate at which ferrite is not precipitated is defined as theupper critical cooling rate.

Here, in the above series of heat treatments, simultaneously with a stepof heating to a temperature range of the Ac₃ point to (Ac₃ point+300)°C. and then cooling to the Ms point, that is, cooling at the uppercritical cooling rate or more, hot forming such as hot stamping may beperformed. As the hot forming, there are bending, drawing, stretching,hole widening, flange forming, and the like. Furthermore, the presentinvention may be applied to a forming method other than press forming,for example, roll forming, as long as a method for cooling the steelsheet is provided simultaneously with or immediately after forming. In acase where the thermal history described above is followed, hot formingmay be repeatedly performed.

As described above, in the present embodiment, both a formed bodyobtained by hot forming and a flat sheet obtained by only a heattreatment are collectively referred to as a “coated steel member”.

Moreover, hot forming or a heat treatment performed on a portion of thesteel to obtain a coated steel member having regions with differentstrengths.

The above series of heat treatments can be performed by any method, andmay be performed by, for example, induction heating hardening,energization heating, infrared heating, or furnace heating.

EXAMPLES

Hereinafter, the present invention will be described more specificallywith reference to examples, but the present invention is not limitedthereto.

First, in producing a coated steel member and a coated steel sheet,steels having the chemical compositions shown in Tables 1 to 4 weremelted to obtain slabs for hot rolling.

Example 1

The obtained slab was hot-rolled and coiled at a temperature of 800° C.or lower to obtain a hot-rolled steel sheet having a thickness of 2.7mm.

Among the kinds of steel shown in Tables 1 to 4, hot-rolled steel sheetsNos. B32 to B46 and b35 to b37 having the steel compositions of SteelNos. A1, A5, A10, A16, A21, and A27 to A31 were subjected to annealing(hot-rolled sheet annealing) in an atmosphere containing 98% of nitrogenat 650° C. for 12 hours.

The hot-rolled steel sheet after the hot rolling or after the hot-rolledsheet annealing was cold-rolled to obtain a cold-rolled steel sheethaving a thickness of 1.6 mm. The cold-rolled steel sheet was subjectedto annealing under the conditions (dew point, temperature, time) shownin Tables 5, 7, and 9. The cold-rolled steel sheet after the annealingwas subjected to Al plating to obtain a coated steel sheet having anAl-based coating. In the plating step, hot-dip Al plating was performedusing a plating bath containing Si and Ni in a total amount of 3.0 to10.0 mass %.

The chemical composition of the coated steel sheet thickness at athickness ¼ position from the surface in the sheet thickness directionof the steel sheet was similar to the chemical composition of the slab.

TABLE 1 Steel Chemical composition (mass %) remainder being Fe andimpurities No. C Si Mn P S N Ti B Nb Cr Ni Cu Invention A1 0.28 0.201.25 0.010 0.0006 0.004 0.030 0.0020 0.06 0.20 0.01 Example A2 0.55 0.150.40 0.007 0.0005 0.003 0.032 0.0026 0.04 0.10 A3 0.36 0.30 1.20 0.0080.0005 0.004 0.040 0.0025 0.05 0.20 A4 0.32 1.60 0.90 0.009 0.0006 0.0030.032 0.0026 0.05 A5 0.42 0.60 0.40 0.008 0.0010 0.002 0.035 0.0021 0.070.15 0.10 A6 0.30 0.40 2.40 0.007 0.0008 0.002 0.026 0.0024 0.06 A7 0.290.25 0.75 0.030 0.0005 0.003 0.038 0.0020 0.04 A8 0.31 0.50 0.60 0.0050.0080 0.002 0.040 0.0025 0.04 0.15 A9 0.33 0.70 0.75 0.009 0.0012 0.0080.032 0.0026 0.05 A10 0.30 0.55 0.70 0.008 0.0010 0.003 0.020 0.00250.04 0.18 A11 0.45 0.63 0.65 0.012 0.0009 0.002 0.080 0.0026 0.05 0.30A12 0.34 0.38 0.85 0.014 0.0011 0.004 0.034 0.0010 0.06 0.18 A13 0.340.41 0.75 0.016 0.0008 0.003 0.022 0.0060 0.04 A14 0.36 0.40 0.97 0.0100.0010 0.004 0.040 0.0025 0.03 0.30 A15 0.37 0.45 0.80 0.008 0.00090.003 0.032 0.0017 0.08 0.23 A16 0.32 0.52 0.78 0.010 0.0009 0.004 0.0400.0025 0.04 0.60 0.10 0.20 A17 0.48 0.20 0.79 0.009 0.0006 0.005 0.0320.0022 0.04 0.60 A18 0.50 0.39 0.65 0.011 0.0008 0.004 0.040 0.0025 0.060.60 A19 0.31 0.40 0.55 0.010 0.0005 0.003 0.032 0.0026 0.07 0.05 A200.45 0.22 0.67 0.007 0.0007 0.006 0.040 0.0028 0.05 A21 0.40 0.51 0.780.008 0.0006 0.003 0.032 0.0026 0.05 0.25 0.23 0.20 A22 0.42 0.35 1.100.005 0.0008 0.004 0.040 0.0022 0.06 A23 0.31 0.63 0.75 0.010 0.00080.004 0.040 0.0020 0.07 A24 0.45 0.43 0.65 0.008 0.0005 0.003 0.0320.0018 0.05 0.20 0.20 A25 0.31 0.47 0.60 0.010 0.0004 0.005 0.040 0.00250.04 A26 0.35 0.26 1.05 0.014 0.0005 0.003 0.032 0.0026 0.04 A27 0.280.15 1.20 0.010 0.0010 0.003 0.030 0.0020 0.05 0.38 0.01 A28 0.30 0.400.80 0.010 0.0008 0.004 0.035 0.0025 0.05 0.14 0.10 0.25 A29 0.35 0.420.60 0.010 0.0009 0.003 0.026 0.0026 0.04 0.12 0.15 0.20 A30 0.50 0.420.40 0.008 0.0005 0.003 0.022 0.0025 0.04 0.12 0.15 0.20 A31 0.34 0.600.60 0.015 0.0020 0.004 0.030 0.0025 0.04 0.40 0.40 0.01

TABLE 2 Transformation Steel Chemical composition (mass %) remainderbeing Fe and impurities point (° C.) Upper critical No. Mo V Ca Al Sn WSb REM Ac3 Ms cooling rate (° C./s) Invention A1 0.01 0.001 0.04 806 39120 Example A2 0.20 0.04 0.01 0.02 771 316 10 A3 0.10 0.01 0.01 791 36120 A4 894 381 20 A5 0.20 0.02 0.01 825 362 20 A6 0.05 0.10 0.003 0.030.05 775 340 10 A7 0.15 0.05 0.18 828 406 30 A8 0.10 0.12 828 404 30 A90.30 0.05 0.15 835 389 30 A10 0.002 0.15 0.15 834 399 30 A11 0.02 0.01829 341 30 A12 0.20 810 380 30 A13 0.02 0.24 816 387 10 A14 0.25 0.300.12 804 368 20 A15 0.15 0.18 804 369 20 A16 0.003 0.16 841 379 20 A170.04 0.18 772 318 20 A18 0.25 787 321 20 A19 0.50 0.002 0.15 827 404 10A20 0.10 0.60 0.20 788 347 30 A21 0.008 0.15 0.25 812 352 30 A22 0.100.60 787 342 20 A23 0.25 0.50 0.14 839 396 30 A24 0.002 0.60 797 339 30A25 0.12 0.20 0.50 831 404 40 A26 0.20 797 374 30 A27 0.01 0.001 0.030.01 0.01 808 388 20 A28 0.20 0.04 0.01 823 394 20 A29 0.20 0.001 0.030.10 0.01 0.01 812 380 20 A30 0.20 0.10 0.04 0.10 0.10 0.01 793 330 20A31 0.20 0.005 0.04 835 373 20

TABLE 3 Steel Chemical composition (mass %) remainder being Fe andimpurities No. C Si Mn P S N Ti B Nb Cr Ni Cu Comparative a1 0.13 0.400.85 0.010 0.0008 0.004 0.040 0.0025 0.04 0.26 0.10 Example a2 0.90 0.200.40 0.005 0.0004 0.002 0.020 0.0020 0.04 0.10 0.05 a3 0.30 0.02 0.350.011 0.0007 0.004 0.060 0.0015 0.04 0.20 0.10 a4 0.45 3.00 1.50 0.0070.0008 0.005 0.032 0.0018 0.06 0.40 0.25 a5 0.36 0.40 0.04 0.009 0.00090.004 0.040 0.0025 0.07 a6 0.38 0.45 5.00 0.008 0.0010 0.003 0.0320.0026 0.06 0.10 a7 0.38 0.52 0.90 0.100 0.0009 0.007 0.040 0.0017 0.040.20 a8 0.43 0.47 0.80 0.010 0.1000 0.002 0.040 0.0025 0.05 0.43 a9 0.390.29 0.85 0.010 0.0010 0.100 0.032 0.0030 0.05 a10 0.45 0.40 0.79 0.0090.0010 0.002 0.001 0.0029 0.05 a11 0.34 0.56 1.00 0.009 0.0008 0.0060.300 0.0026 0.04 0.20 a12 0.34 0.45 0.69 0.008 0.0012 0.004 0.0210.0002 0.04 0.30 0.10 a13 0.40 0.52 0.74 0.010 0.0009 0.002 0.022 0.03000.05 a14 0.36 0.50 1.00 0.009 0.0008 0.004 0.021 0.0025 0.01 0.20 a150.37 0.25 0.45 0.010 0.0009 0.006 0.035 0.0020 0.25 a16 0.41 0.39 1.100.011 0.0006 0.003 0.022 0.0015 0.05 2.00 0.30 a17 0.42 0.40 0.81 0.0080.0009 0.005 0.021 0.0025 0.06 a18 0.39 0.66 1.00 0.015 0.0005 0.0070.040 0.0018 0.04 0.25 a19 0.42 0.51 0.76 0.008 0.0008 0.005 0.0260.0026 0.07 0.22

TABLE 4 Transformation Steel Chemical composition (mass %) remainderbeing Fe and impurities point (° C.) Upper critical No. Mo V Ca Al Sn WSb REM Ac3 Ms cooling rate (° C./s) Comparative a1 0.03 0.03 867 461 40Example a2 0.10 0.002 0.10 0.03 770 172 10 a3 0.10 0.10 0.30 0.35 824415 70 a4 915 278 20 a5 0.25 0.005 0.25 831 403 100 a6 0.05 0.18 765 20910 a7 0.01 0.15 878 359 30 a8 0.30 0.25 819 336 30 a9 0.20 0.003 0.20794 364 20 a10 0.10 783 338 50 a11 0.01 0.04 0.22 0.01 910 372 30 a12821 383 50 a13 0.25 0.30 0.04 806 362 20 a14 0.30 0.03 803 365 20 a150.04 0.10 0.32 807 382 50 a16 0.003 0.25 0.24 851 301 20 a17 1.50 0.450.35 793 349 10 a18 0.06 2.00 0.35 824 346 30 a19 0.30 0.35 0.20 1.50812 348 20

Regarding the obtained coated steel sheet, a lower layer containing 3mass % or more and less than 70 mass % of Al and an upper layercontaining 70 mass % or more and 95 mass % or less of Al weredetermined, and the total amount of Si and Ni in the upper layer, themaximum C content from the interface between the steel sheet and thecoating to a depth of 20 μm on the steel sheet side, and the Cr contentin the lower layer were evaluated.

Specifically, the maximum values of the total amount of Si and Ni in theupper layer and the Cr content in the lower layer in the Al-basedcoating were obtained by performing glow discharge emission analysis(GDS) in the sheet thickness direction from the surface of the coatedsteel sheet as follows. When obtaining the maximum values of the amountof Si and Ni of the upper layer and the Cr content of the lower layer, aregion where the Al content was 3 mass % or more and the Fe content wasless than 95 mass % was determined to be the Al-based coating, and aregion where the Fe content was 95 mass % or more was determined to bethe steel sheet. In addition, in the Al-based coating, a region wherethe Al content was 70 mass % or more was determined to be the upperlayer, and a region where the Al content was less than 70% wasdetermined to be the lower layer. Glow discharge emission analysis (GDS)was performed at an approximately ¼ position of width (short direction)from an width direction end portion of the coated steel member in thesheet thickness direction from the surface of the coated steel sheet toobtain the Si content and the Ni content in the upper layer, and the sumof the Si content and the Ni content at a position where the totalcontent was the largest was used as the total amount of Si and Ni of theupper layer in the upper layer. Furthermore, glow discharge emissionanalysis (GDS) was performed at an approximately ¼ position of width(short direction) from the width direction end portion of the coatedsteel sheet in the sheet thickness direction from the surface of thecoated steel sheet to measure the maximum value of the Cr content in thelower layer. This measurement was performed a total of five times, andthe values obtained in each measurement were averaged to obtain themaximum values of the total amount of Si and Ni of the upper layer inthe upper layer and the Cr content of the lower layer.

The evaluation results are shown in Tables 5 to 10.

In addition, a heat treatment of heating the coated steel sheet to 920°C. at a temperature rising rate of 5.0° C./s and cooling the coatedsteel sheet to an Ms point or lower at 50° C./s was performed to obtaina coated steel member.

The obtained coated steel member was cut out and subjected to glowdischarge emission analysis (GDS), a tensile test, a bending test, and aCharpy impact test by the following methods, and the maximum C contentin the coating region containing 30 mass % or more of Al, the maximum Ccontent in the region containing 3 mass % or more and less than 30 mass% of Al, the maximum C content in the range from the interface betweenthe steel sheet substrate and the coating to the depth of 10 μm on thesteel sheet substrate side, the maximum Cr content in the high Alcontent region, tensile strength, bending angle, and impact value wereevaluated. The evaluation results are shown in Tables 5 to 10.

Maximum C Contents in High Al Content Region and Low Al Content Region

GDS was performed in the sheet thickness direction from the surface ofthe coated steel member to examine the Al content and the C content. TheGDS measurement was performed at five points at random at a ¼ positionof width (short direction) from the width direction end portion of thecoated steel member. As a result of the measurement, a region where theAl content was 30 mass % or more was determined to be the high Alcontent region, a region where the Al content was 3 mass % or more andless than 30 mass % was determined to be the low Al content region, themaximum C content in the high Al content region and the maximum Ccontent in the low Al content region were obtained, the measurement wasperformed five times, and the average values thereof were used to obtainthe maximum C content in the high Al content region and the maximum Ccontent in the low Al content region.

Maximum C Content in Range from Surface of Steel Sheet Substrate toDepth of 10 μm

GDS was performed in the sheet thickness direction from the surface ofthe coated steel member to examine the Fe content and the C content. Inthe GDS measurement, at the ¼ position of width (short direction) fromthe width direction end portion of the coated steel member, the maximumC concentration in the range from the surface of the steel sheetsubstrate to a depth of 10 μm was obtained. The measurement wasperformed five times, and the average values thereof were used to obtainthe maximum C content in the range from the surface of the steel sheetsubstrate to the depth of 10 μm. Here, the surface of the steel sheetsubstrate is set to a depth position at which the Fe content was 95% ormore.

Maximum Cr Content in High Al Content Region

GDS was performed in the sheet thickness direction from the surface ofthe coated steel member to examine the Al content and the Cr content.The GDS measurement was performed at five points at random at a ¼position of width (short direction) from the width direction end portionof the coated steel member. As a result of the measurement, a regionwhere the Al content was 30 mass % or more was determined to be the highAl content region, a region where the Al content was 3 mass % or moreand less than 30 mass % was determined to be the low Al content region,and the maximum Cr content in the high Al content region was obtained.The measurement was performed five times, and the average value thereofwas used to obtain the maximum Cr content in the high Al content region.

Tensile Strength

A tensile test was performed in accordance with ASTM Standard E8. Aftergrinding a soaked portion of the coated steel member to a thickness of1.2 mm, a half-sized sheet-shaped test piece of ASTM standard E8(parallel portion length: 32 mm, parallel portion sheet width: 6.25 mm)was collected so that the test direction was parallel to the rollingdirection. Then, a room temperature tensile test was performed at astrain rate of 3 mm/min to measure the tensile strength (maximumstrength). In this example, a case where the tensile strength exceeded1500 MPa was evaluated as having excellent strength.

Bending Angle

A bending test was performed in accordance with the regulations ofVDA238-100. A bending test piece of 60 mm parallel and 30 mmperpendicular to the rolling direction was collected from the soakedportion of the coated steel member. A bending punch was aligned so as tobe perpendicular to the rolling direction, and a bending angle at themaximum load was measured. Since the bending angle correlates with thestrength, in this example, a case of having a bending angle of 60degrees when the tensile strength was less than 2100 MPa and a bendingangle of more than 50 degrees when the tensile strength was 2100 MPa ormore was evaluated as having better bendability than in the related art.

Impact Value

A Charpy impact test was performed in accordance with JIS Z 2242: 2018.A 2 mm V-notch test piece was prepared by cutting out 55 mm in therolling direction and 10 mm in the direction perpendicular thereto fromthe soaked portion of the coated steel member and laminating threecut-out portions, a Charpy impact test was performed thereon at a testtemperature of −40° C., and an impact value (J/cm²) was obtained bydividing the absorbed energy vE by the cross-sectional area of the testpiece at the bottom of the notch. Since the impact value correlates withthe strength, in this example, a case of having an impact value of 35J/cm² or more when the tensile strength was less than 2100 MPa and animpact value of 20 J/cm² or more when the tensile strength was 2100 MPaor more was evaluated as having excellent toughness.

TABLE 5 Production conditions Hot-rolled sheet annealing Annealing N₂Dew Coating Steel concentration Temperature Time point Temperature TimeKind Si + Ni Symbol No. % ° C. hr ° C. ° C. sec — (mass %) Invention B1A1 — — — 5 760 60 Hot-dip plating 10.0 Example B2 A2 — — — 5 760 60Hot-dip plating 10.0 B3 A3 — — — 5 760 60 Hot-dip plating 10.0 B4 A4 — —— 5 760 60 Hot-dip plating 10.0 B5 A5 — — — 5 760 60 Hot-dip plating10.0 B6 A6 — — — 5 760 60 Hot-dip plating 10.0 B7 A7 — — — 5 760 60Hot-dip plating 10.0 B8 A8 — — — 5 760 60 Hot-dip plating 10.0 B9 A9 — —— 5 760 60 Hot-dip plating 10.0 B10 A10 — — — 5 760 60 Hot-dip plating10.0 B11 A11 — — — 5 760 60 Hot-dip plating 10.0 B12 A12 — — — 5 760 60Hot-dip plating 10.0 B13 A13 — — — 5 760 60 Hot-dip plating 10.0 B14 A14— — — 5 760 60 Hot-dip plating 10.0 B15 A15 — — — 5 760 60 Hot-dipplating 10.0 B16 A16 — — — 5 760 60 Hot-dip plating 10.0 B17 A17 — — — 5760 60 Hot-dip plating 10.0 B18 A18 — — — 5 760 60 Hot-dip plating 10.0B19 A19 — — — 5 760 60 Hot-dip plating 10.0 B20 A20 — — — 5 760 60Hot-dip plating 10.0 B21 A21 — — — 5 760 60 Hot-dip plating 10.0 B22 A22— — — 5 760 60 Hot-dip plating 10.0 B23 A23 — — — 5 760 60 Hot-dipplating 10.0 Coated steel sheet Production conditions Amount of Si + Ccontent in Maximum Cr Heat treatment Ni in upper surface layer ofcontent in lower Temperature Heating layer steel sheet layer rising ratetemperature Symbol (mass %) (mass %) (mass %) (° C./s) (° C.) InventionB1 8.8 0.17 0.22 5 920 Example B2 8.8 0.39 0.00 5 920 B3 8.9 0.24 0.00 5920 B4 8.5 0.20 0.00 5 920 B5 8.9 0.30 0.15 5 920 B6 8.7 0.19 0.00 5 920B7 8.7 0.17 0.00 5 920 B8 9.0 0.20 0.00 5 920 B9 9.0 0.21 0.00 5 920 B108.8 0.18 0.20 5 920 B11 8.7 0.31 0.00 5 920 B12 8.7 0.22 0.00 5 920 B138.6 0.22 0.00 5 920 B14 8.8 0.23 0.00 5 920 B15 8.9 0.25 0.00 5 920 B168.8 0.20 0.70 5 920 B17 8.7 0.34 0.00 5 920 B18 8.9 0.36 0.00 5 920 B199.0 0.20 0.00 5 920 B20 8.7 0.32 0.00 5 920 B21 8.7 0.26 0.27 5 920 B228.8 0.30 0.00 5 920 B23 8.9 0.20 0.00 5 920

TABLE 6 Coated steel member Production Maximum C conditions Maximum CMaximum C concentration Maximum Cr Heat treatment content in content inin surface content in Cooling Upper high Al low Al layer of high Al rateto Ms critical content content steel sheet content Tensile BendingImpact Steel or lower cooling rate region region substrate regionstrength angle value Symbol No. (° C./s) (° C./s) (mass %) (mass %)(mass %) (mass %) (MPa) (°) (J/cm²) Invention B1 A1 50 20 0.05 0.08 0.180.12 1705 71 53 Example B2 A2 50 10 0.12 0.19 0.40 0.00 2873 52 22 B3 A350 20 0.06 0.12 0.24 0.00 2070 61 38 B4 A4 50 20 0.06 0.10 0.20 0.001855 65 42 B5 A5 50 20 0.08 0.13 0.29 0.09 2269 57 28 B6 A6 50 10 0.050.08 0.19 0.00 1908 63 38 B7 A7 50 30 0.05 0.09 0.17 0.00 1699 70 50 B8A8 50 30 0.05 0.08 0.20 0.00 1777 66 45 B9 A9 50 30 0.05 0.10 0.22 0.001881 63 39 B10 A10 50 30 0.05 0.08 0.20 0.08 1736 68 46 B11 A11 50 300.07 0.15 0.32 0.00 2433 55 25 B12 A12 50 30 0.07 0.09 0.22 0.00 1941 6238 B13 A13 50 10 0.07 0.10 0.23 0.00 1928 62 39 B14 A14 50 20 0.06 0.100.24 0.00 2046 62 37 B15 A15 50 20 0.07 0.10 0.26 0.00 2072 61 37 B16A16 50 20 0.06 0.09 0.22 0.40 1842 64 45 B17 A17 50 20 0.09 0.15 0.340.00 2584 54 24 B18 A18 50 20 0.09 0.17 0.35 0.00 2659 53 23 B19 A19 5010 0.06 0.08 0.21 0.00 1769 72 46 B20 A20 50 30 0.09 0.15 0.32 0.00 243156 25 B21 A21 50 30 0.08 0.13 0.26 0.16 2208 58 28 B22 A22 50 20 0.080.14 0.29 0.00 2334 57 27 B23 A23 50 30 0.06 0.08 0.20 0.00 1790 65 44

TABLE 7 Production conditions Hot-rolled sheet annealing Annealing N₂Dew Coating Steel concentration Temperature Time point Temperature TimeKind Si + Ni Symbol No. % ° C. hr ° C. ° C. sec — (mass %) Invention B24A24 — — — 5 760 60 Hot-dip plating 10.0 Example B25 A25 — — — 5 760 60Hot-dip plating 10.0 B26 A26 — — — 5 760 60 Hot-dip plating 10.0 B27 A27— — — 5 760 60 Hot-dip plating 10.0 B28 A28 — — — 5 760 60 Hot-dipplating 10.0 B29 A29 — — — 5 760 60 Hot-dip plating 10.0 B30 A30 — — — 5760 60 Hot-dip plating 10.0 B31 A31 — — — 5 760 60 Hot-dip plating 10.0B32 A1 98 650 12 −30 740 15 Hot-dip plating 3.0 B33 A5 98 650 12 −30 74015 Hot-dip plating 3.0 B34 A10 98 650 12 −30 740 15 Hot-dip plating 3.0B35 A16 98 650 12 −30 740 15 Hot-dip plating 3.0 B36 A21 98 650 12 −30740 15 Hot-dip plating 3.0 B37 A27 98 650 12 −30 740 15 Hot-dip plating3.0 B38 A28 98 650 12 −30 740 15 Hot-dip plating 3.0 B39 A29 98 650 12−30 740 15 Hot-dip plating 3.0 B40 A30 98 650 12 −30 740 15 Hot-dipplating 3.0 B41 A31 98 650 12 −30 740 15 Hot-dip plating 3.0 B42 A27 98650 12 5 760 60 Hot-dip plating 10.0 B43 A28 98 650 12 5 760 60 Hot-dipplating 10.0 B44 A29 98 650 12 5 760 60 Hot-dip plating 10.0 B45 A30 98650 12 5 760 60 Hot-dip plating 10.0 B46 A31 98 650 12 5 760 60 Hot-dipplating 10.0 Coated steel sheet Production conditions Amount of Si + Ccontent in Maximum Cr Heat treatment Ni in upper surface layer ofcontent in lower Temperature Heating layer steel sheet layer rising ratetemperature Symbol (mass %) (mass %) (mass %) (° C./s) (° C.) InventionB24 9.0 0.31 0.00 5 920 Example B25 9.0 0.20 0.00 5 920 B26 8.5 0.220.00 5 920 B27 8.7 0.20 0.35 5 920 B28 8.9 0.21 0.16 5 920 B29 8.9 0.260.12 5 920 B30 8.8 0.38 0.11 5 920 B31 8.7 0.26 0.33 5 920 B32 2.0 0.160.28 5 920 B33 2.0 0.30 0.22 5 920 B34 2.1 0.17 0.25 5 920 B35 2.0 0.190.75 5 920 B36 2.0 0.26 0.33 5 920 B37 2.1 0.19 0.52 5 920 B38 2.3 0.220.25 5 920 B39 2.2 0.25 0.25 5 920 B40 1.9 0.36 0.23 5 920 B41 1.9 0.260.51 5 920 B42 8.8 0.17 0.50 5 920 B43 9.0 0.19 0.23 5 920 B44 8.9 0.230.21 5 920 B45 8.7 0.35 0.21 5 920 B46 8.8 0.23 0.52 5 920

TABLE 8 Coated steel member Production Maximum C conditions Maximum CMaximum C concentration Maximum Cr Heat treatment content in content inin surface content in Cooling Upper high Al low Al layer of high Al rateto Ms critical content content steel sheet content Tensile BendingImpact Steel or lower cooling rate region region substrate regionstrength angle value Symbol No. (° C./s) (° C./s) (mass %) (mass %)(mass %) (mass %) (MPa) (°) (J/cm²) Invention B24 A24 50 30 0.08 0.150.31 0.00 2432 56 25 Example B25 A25 50 40 0.06 0.07 0.20 0.00 1778 7144 B26 A26 50 30 0.07 0.11 0.22 0.00 2007 62 38 B27 A27 50 20 0.05 0.090.19 0.20 1712 70 52 B28 A28 50 20 0.07 0.10 0.21 0.08 1763 66 48 B29A29 50 20 0.08 0.12 0.25 0.07 1968 62 39 B30 A30 50 20 0.11 0.18 0.380.07 2642 55 26 B31 A31 50 20 0.08 0.12 0.26 0.19 1749 66 47 B32 A1 5020 0.05 0.08 0.18 0.18 1703 70 53 B33 A5 50 20 0.07 0.12 0.28 0.14 226757 29 B34 A10 50 30 0.05 0.08 0.19 0.17 1735 68 46 B35 A16 50 20 0.060.08 0.22 0.52 1842 65 46 B36 A21 50 30 0.07 0.13 0.26 0.22 2210 58 29B37 A27 50 20 0.05 0.08 0.20 0.33 1711 71 53 B38 A28 50 20 0.06 0.090.22 0.12 1765 67 47 B39 A29 50 20 0.07 0.12 0.26 0.11 1970 62 40 B40A30 50 20 0.10 0.19 0.38 0.12 2644 55 25 B41 A31 50 20 0.08 0.12 0.260.34 1751 65 48 B42 A27 50 20 0.03 0.07 0.17 0.33 1696 81 54 B43 A28 5020 0.04 0.08 0.19 0.12 1755 72 49 B44 A29 50 20 0.05 0.10 0.23 0.11 195966 40 B45 A30 50 20 0.07 0.16 0.36 0.11 2635 59 27 B46 A31 50 20 0.050.10 0.23 0.34 1752 69 47

TABLE 9 Production conditions Hot-rolled sheet annealing Annealing N₂Dew Coating Steel concentration Temperature Time point Temperature TimeKind Si + Ni Symbol No. % ° C. hr ° C. ° C. sec — (mass %) Comparativeb1 a1 — — — −30 740 15 Hot-dip plating 3.0 Example b2 a2 — — — −30 74015 Hot-dip plating 3.0 b3 a3 — — — −30 740 15 Hot-dip plating 3.0 b4 a4— — — −30 740 15 Hot-dip plating 3.0 b5 a5 — — — −30 740 15 Hot-dipplating 3.0 b6 a6 — — — −30 740 15 Hot-dip plating 3.0 b7 a7 — — — −30740 15 Hot-dip plating 3.0 b8 a8 — — — −30 740 15 Hot-dip plating 3.0 b9a9 — — — −30 740 15 Hot-dip plating 3.0 b10 a10 — — — −30 740 15 Hot-dipplating 3.0 b11 a11 — — — −30 740 15 Hot-dip plating 3.0 b12 a12 — — —−30 740 15 Hot-dip plating 3.0 b13 a13 — — — −30 740 15 Hot-dip plating3.0 b14 a14 — — — −30 740 15 Hot-dip plating 3.0 b15 a15 — — — −30 74015 Hot-dip plating 3.0 b16 a16 — — — −30 740 15 Hot-dip plating 3.0 b17a17 — — — −30 740 15 Hot-dip plating 3.0 b18 a18 — — — −30 740 15Hot-dip plating 3.0 b19 a19 — — — −30 740 15 Hot-dip plating 3.0 b20 A27— — — −30 740 15 Hot-dip plating 3.0 b21 A28 — — — −30 740 15 Hot-dipplating 3.0 b22 A29 — — — −30 740 15 Hot-dip plating 3.0 b23 A30 — — —−30 740 15 Hot-dip plating 3.0 b24 A31 — — — −30 740 15 Hot-dip plating3.0 b25 A27 — — — 5 760 60 Hot-dip plating 3.0 b26 A28 — — — 5 760 60Hot-dip plating 3.0 b27 A29 — — — 5 760 60 Hot-dip plating 3.0 b28 A30 —— — 5 760 60 Hot-dip plating 3.0 b29 A31 — — — 5 760 60 Hot-dip plating3.0 b30 A27 — — — −30 740 15 Hot-dip plating 10.0 b31 A28 — — — −30 74015 Hot-dip plating 10.0 b32 A29 — — — −30 740 15 Hot-dip plating 10.0b33 A30 — — — −30 740 15 Hot-dip plating 10.0 b34 A31 — — — −30 740 15Hot-dip plating 10.0 b35 a2 98 650 12 5 760 60 Hot-dip plating 10.0 b36a6 98 650 12 5 760 60 Hot-dip plating 10.0 b37 a8 98 650 12 5 760 60Hot-dip plating 10.0 Coated steel sheet Production conditions Amount ofSi + C content in Maximum Cr Heat treatment Ni in upper surface layer ofcontent in lower Temperature Heating layer steel sheet layer rising ratetemperature Symbol (mass %) (mass %) (mass %) (° C./s) (° C.)Comparative b1 2.2 0.11 0.29 5 920 Example b2 2.1 0.88 0.09 5 920 b3 2.30.28 0.22 5 920 b4 2.2 0.42 0.45 5 920 b5 2.2 0.34 0.00 5 920 b6 2.30.36 0.00 5 920 b7 2.4 0.36 0.00 5 920 b8 2.2 0.42 0.49 5 920 b9 2.20.37 0.00 5 920 b10 2.1 0.43 0.00 5 920 b11 2.1 0.33 0.00 5 920 b12 2.30.32 0.31 5 920 b13 2.4 0.39 0.00 5 920 b14 2.1 0.34 0.00 5 920 b15 2.00.35 0.00 5 920 b16 2.0 0.40 2.30 5 920 b17 2.0 0.40 0.00 5 920 b18 2.10.37 0.00 5 920 b19 1.9 0.41 0.22 5 920 b20 2.0 0.27 0.40 5 920 b21 2.20.29 0.15 5 920 b22 2.1 0.34 0.12 5 920 b23 1.9 0.49 0.13 5 920 b24 2.00.34 0.40 5 920 b25 2.1 0.19 0.38 5 920 b26 2.1 0.21 0.15 5 920 b27 2.00.25 0.12 5 920 b28 1.9 0.38 0.12 5 920 b29 2.0 0.25 0.37 5 920 b30 8.70.26 0.39 5 920 b31 8.8 0.28 0.14 5 920 b32 8.9 0.33 0.11 5 920 b33 8.80.48 0.12 5 920 b34 8.7 0.33 0.39 5 920 b35 8.8 0.64 0.28 5 920 b36 9.00.25 0.00 5 920 b37 8.9 0.27 0.60 5 920

TABLE 10 Coated steel member Production Maximum C conditions Maximum CMaximum C concentration Heat treatment content in content in in surfaceMaximum Cr Cooling Upper high Al low Al layer of content in rate to Mscritical content content steel sheet high Al Tensile Bending ImpactSteel or lower cooling rate region region substrate content regionstrength angle value Symbol No. (° C./s) (° C./s) (mass %) (mass %)(mass %) (mass %) (MPa) (°) (J/cm²) Comparative b1 a1 50 40 0.07 0.090.12 0.15 964 92 90 Example b2 a2 50 10 0.26 0.41 0.88 0.04 2720 30 16b3 a3 50 70 0.13 0.16 0.28 0.12 1121 85 84 b4 a4 50 20 0.15 0.23 0.430.24 2518 35 15 b5 a5 50 100 0.14 0.18 0.35 0.00 1070 87 88 b6 a6 50 100.16 0.20 0.36 0.00 2380 28 8 b7 a7 50 30 0.14 0.19 0.37 0.00 2131 39 9b8 a8 50 30 0.15 0.23 0.41 0.28 2353 29 12 b9 a9 50 20 0.13 0.21 0.370.00 2168 38 16 b10 a10 50 50 0.15 0.24 0.43 0.00 2447 35 18 b11 a11 5030 0.14 0.19 0.32 0.00 1150 85 85 b12 a12 50 50 0.15 0.19 0.32 0.18 192643 25 b13 a13 50 20 0.15 0.21 0.39 0.00 2210 38 14 b14 a14 50 20 0.130.19 0.34 0.00 2044 40 28 b15 a15 50 50 0.14 0.21 0.34 0.00 2038 39 27b16 a16 50 20 0.16 0.21 0.39 1.30 2292 38 18 b17 a17 50 10 0.16 0.220.40 0.00 2310 36 17 b18 a18 50 30 0.14 0.21 0.37 0.00 2188 38 15 b19a19 50 20 0.15 0.23 0.39 0.14 2304 37 14 b20 A27 50 20 0.14 0.16 0.270.26 1750 50 50 b21 A28 50 20 0.15 0.18 0.29 0.08 1809 49 48 b22 A29 5020 0.17 0.20 0.34 0.07 1998 44 40 b23 A30 50 20 0.25 0.32 0.50 0.07 267838 25 b24 A31 50 20 0.18 0.20 0.34 0.07 2012 42 38 b25 A27 50 20 0.110.13 0.20 0.26 1732 58 51 b26 A28 50 20 0.11 0.14 0.22 0.07 1790 57 50b27 A29 50 20 0.11 0.17 0.26 0.06 1989 53 42 b28 A30 50 20 0.15 0.250.38 0.06 2659 47 27 b29 A31 50 20 0.12 0.17 0.26 0.06 2001 51 40 b30A27 50 20 0.13 0.17 0.26 0.25 1746 51 49 b31 A28 50 20 0.16 0.17 0.280.08 1805 48 50 b32 A29 50 20 0.16 0.21 0.33 0.06 1993 45 40 b33 A30 5020 0.24 0.34 0.48 0.06 2665 39 26 b34 A31 50 20 0.16 0.22 0.33 0.06 200844 39 b35 a2 50 10 0.21 0.34 0.69 0.09 2680 44 17 b36 a6 50 10 0.07 0.120.28 0.00 2346 48 10 b37 a8 50 30 0.08 0.14 0.29 0.36 2320 46 12

As shown in Tables 5 to 10, Invention Examples B1 to B46 satisfying theranges of the present invention obtained good results in terms of boththe structure and properties, but Comparative Examples b1 to b37 notsatisfying the ranges of the present invention obtained results notsatisfying at least one of the structure and properties.

Example 2

Among the kinds of steel shown in Tables 1 to 4, slabs having the steelcompositions of Steel Nos. A27 to A31 were hot-rolled to obtainhot-rolled steel sheets having a thickness of 2.7 mm. The hot-rolledsteel sheets were subjected to hot-rolled sheet annealing and annealingas shown in Tables 9 to 10 and further subjected to plating by beingimmersed in an Al plating bath containing Si and Ni shown in Tables 11and 12 to obtain coated steel sheets.

In the obtained coated steel sheets, as in Example 1, GDS was used todetermine the lower layer containing 3 mass % or more and less than 70mass % of Al and the upper layer containing 70 mass % or more and 95mass % or less of Al, and the total amount of Si and Ni in the upperlayer, the maximum C content in the range from the surface of the steelsheet to the depth of 20 μm, and the Cr content in the lower layer wereevaluated. The evaluation results are shown in Tables 9 and 10.

TABLE 11 Coated steel sheet Production conditions Amount of Maximum CHot-rolled sheet annealing Annealing Si + Ni content in Cr content N₂Dew Coating in upper surface layer in lower Steel concentrationTemperature Time point Temperature Time Si + Al layer of steel sheetlayer Symbol No. % ° C. hr ° C. ° C. sec (mass %) (mass %) (mass %)(mass %) Invention D1 A27 — — — 5 760 60 10.0 8.7 0.20 0.35 Example D2A27 50 350 3 2 840 100  12.0 10.2 0.19 0.34 D3 A27 98 650 12 5 760 603.0 2.0 0.18 0.51 D4 A27 95 700 7 2 840 100  2.0 1.2 0.17 0.50 D5 A27 98650 15 −30 740 15 3.0 2.1 0.19 0.52 D6 A27 95 680 10 −20 800 20 2.0 1.10.20 0.54 D7 A27 98 650 12 −30 740 15 10.0 8.8 0.20 0.50 D8 A27 96 700 8−15 720 20 12.5 10.8 0.19 0.48 D9 A27 95 780 6 −20 750 20 9.5 8.2 0.180.51 D10 A27 98 650 12 5 760 60 10.0 8.8 0.17 0.50 D11 A27 98 700 8 2770 50 10.0 8.9 0.17 0.52 D12 A27 98 650 20 — — — 10.0 8.8 0.21 0.58 D13A28 — — — 5 760 60 10.0 8.9 0.21 0.16 D14 A28 98 650 12 5 760 60 3.0 2.10.20 0.25 D15 A28 98 650 12 −30 740 15 3.0 2.3 0.22 0.25 D16 A28 98 65012 −30 740 15 10.0 8.8 0.22 0.23 D17 A28 95 780 6 −20 750 20 9.5 8.30.20 0.25 D18 A28 98 650 12 5 760 60 10.0 9.0 0.19 0.23 D19 A28 98 65020 — — — 10.0 8.8 0.22 0.30 D20 A29 — — — 5 760 60 10.0 8.9 0.26 0.12D21 A29 98 650 12 5 760 60 3.0 2.2 0.24 0.24 D22 A29 98 650 12 −30 74015 3.0 2.0 0.25 0.25 D23 A29 98 650 12 −30 740 15 10.0 8.8 0.25 0.22 D24A29 95 780 6 −20 750 20 9.5 8.3 0.24 0.24 D25 A29 98 650 12 5 760 6010.0 8.9 0.23 0.21 D26 A29 98 650 20 — — — 10.0 8.8 0.26 0.28 D27 A30 —— — 5 760 60 10.0 8.8 0.38 0.11 D28 A30 98 650 12 5 760 60 3.0 2.1 0.350.23 D29 A30 98 650 12 −30 740 15 3.0 1.9 0.36 0.23 D30 A30 98 650 12−30 740 15 10.0 8.8 0.37 0.22 D31 A30 95 780 6 −20 750 20 9.5 8.2 0.360.24 D32 A30 98 650 12 5 760 60 10.0 8.7 0.35 0.21 D33 A30 98 650 20 — —— 10.0 8.8 0.38 0.30 D34 A31 — — — 5 760 60 10.0 8.7 0.26 0.33 D35 A3198 650 12 5 760 60 3.0 2.1 0.24 0.50 D36 A31 98 650 12 −30 740 15 3.01.9 0.26 0.51 D37 A31 98 650 12 −30 740 15 10.0 8.7 0.25 0.49 D38 A31 95780 6 −20 750 20 9.5 8.3 0.24 0.51 D39 A31 98 650 12 5 760 60 10.0 8.80.23 0.52 D40 A31 98 650 20 — — — 10.0 8.7 0.26 0.59

TABLE 12 Coated steel sheet Production conditions Amount of Maximum CHot-rolled sheet annealing Annealing Si + Ni content in Cr content N₂Dew Coating in upper surface layer in lower Steel concentrationTemperature Time point Temperature Time Si + Ni layer of steel sheetlayer Symbol No. % ° C. hr ° C. ° C. sec (mass %) (mass %) (mass %)(mass %) Comparative d1 A27 — — — −30 740 15 3.0 2.0 0.27 0.40 Exampled2 A27 50 350 3 −30 680 0 2.0 1.2 0.26 0.40 d3 A27 — — — 5 720 60 3.02.1 0.19 0.38 d4 A27 50 350 3 2 740 100 2.0 1.1 0.20 0.34 d5 A27 — — —−30 720 15 10.0 8.7 0.26 0.39 d6 A27 50 350 3 −15 680 0 12.0 11.0 0.260.40 d7 A27 50 550 7 −30 740 15 3.0 2.0 0.25 0.41 d8 A27 90 350 7 −30740 15 3.0 2.0 0.25 0.42 d9 A27 90 550 3 −30 740 15 3.0 2.0 0.24 0.42d10 A28 — — — −30 740 15 3.0 2.2 0.29 0.15 d11 A28 — — — 5 760 60 3.02.1 0.21 0.15 d12 A28 — — — −30 740 15 10.0 8.8 0.28 0.14 d13 A28 50 5507 −30 740 15 3.0 2.1 0.28 0.15 d14 A28 90 350 7 −30 740 15 3.0 2.1 0.280.15 d15 A28 90 550 3 −30 740 15 3.0 2.1 0.27 0.16 d16 A29 — — — −30 74015 3.0 2.1 0.34 0.12 d17 A29 — — — 5 760 60 3.0 2.0 0.25 0.12 d18 A29 —— — −30 740 15 10.0 8.9 0.33 0.11 d19 A29 50 550 7 −30 740 15 3.0 2.10.32 0.12 d20 A29 90 350 7 −30 740 15 3.0 2.1 0.33 0.13 d21 A29 90 550 3−30 740 15 3.0 2.1 0.32 0.12 d22 A30 — — — −30 740 15 3.0 1.9 0.49 0.13d23 A30 — — — 5 760 60 3.0 1.9 0.38 0.12 d24 A30 — — — −30 740 15 10.08.8 0.48 0.12 d25 A30 50 550 7 −30 740 15 3.0 1.9 0.46 0.13 d26 A30 90350 7 −30 740 15 3.0 1.9 0.47 0.12 d27 A30 90 550 3 −30 740 15 3.0 1.90.47 0.13 d28 A31 — — — −30 740 15 3.0 2.0 0.34 0.40 d29 A31 — — — 5 76060 3.0 2.0 0.25 0.37 d30 A31 — — — −30 740 15 10.0 8.7 0.33 0.39 d31 A3150 550 7 −30 740 15 3.0 1.9 0.33 0.40 d32 A31 90 350 7 −30 740 15 3.01.9 0.32 0.41 d33 A31 90 550 3 −30 740 15 3.0 2.0 0.33 0.42

As can be seen from Tables 11 to 12, in Invention Examples D1 to D40satisfying the ranges of the present invention, coated steel sheetshaving a predetermined chemical composition and a structure wereobtained. On the other hand, Comparative Examples d1 to d33 notsatisfying the ranges of the present invention obtained results notsatisfying at least one of the target structures.

Example 3

The coated steel sheets shown in Tables 13 to 16 were subjected to theheat treatments shown in Tables 13 to 16 to produce coated steel members(E1 to E35 and e1 to e45).

The obtained coated steel members were cut out and subjected to glowdischarge emission analysis (GDS), a tensile test, a bending test, and aCharpy impact test by the following methods in the same method as inExample 1, and the maximum C content in the coating region containing30% or more of Al, the maximum C content in the region containing 3 mass% or more and less than 30 mass % of Al, the maximum C content in therange from the surface of the steel sheet substrate to the depth of 10μm, the maximum Cr content in the high Al content region, tensilestrength, bending angle, and impact value were evaluated. The evaluationresults are shown in Tables 13 to 16.

TABLE 13 Coated steel sheet Production conditions Maximum C content Heattreatment Amount of Si + Ni in surface layer Cr content TemperatureHeating Cooling rate to Ms Steel in upper layer of steel sheet in lowerlayer rising rate temperature or lower Symbol No. (mass %) (mass %)(mass %) (° C./s) (° C.) (° C./s) Invention E1 A27 8.7 0.20 0.35 5 92050 Example E2 A27 10.2 0.19 0.34 5 920 50 E3 A27 2.0 0.18 0.51 5 920 50E4 A27 1.2 0.17 0.50 5 920 50 E5 A27 2.1 0.19 0.52 5 920 50 E6 A27 1.10.20 0.54 5 920 50 E7 A27 8.8 0.20 0.50 5 920 50 E8 A27 10.8 0.19 0.48 5920 50 E9 A27 8.2 0.18 0.51 5 920 50 E10 A27 8.8 0.17 0.50 5 920 50 E11A27 8.9 0.17 0.52 5 920 50 E12 A28 8.9 0.21 0.16 5 920 50 E13 A28 2.10.20 0.25 5 920 50 E14 A28 2.3 0.22 0.25 5 920 50 E15 A28 8.8 0.22 0.235 920 50 E16 A28 8.3 0.20 0.25 5 920 50 E17 A28 9.0 0.19 0.23 5 920 50E18 A29 8.9 0.26 0.12 5 920 50 E19 A29 2.2 0.24 0.24 5 920 50 E20 A292.0 0.25 0.25 5 920 50 E21 A29 8.8 0.25 0.22 5 920 50 E22 A29 8.3 0.240.24 5 920 50 E23 A29 8.9 0.23 0.21 5 920 50 E24 A30 8.8 0.38 0.11 5 92050 E25 A30 2.1 0.36 0.23 5 920 50 E26 A30 1.9 0.38 0.24 5 920 50 E27 A308.8 0.37 0.22 5 920 50 E28 A30 8.2 0.36 0.24 5 920 50 E29 A30 8.7 0.350.21 5 920 50 E30 A31 8.7 0.26 0.33 5 920 50 E31 A31 2.1 0.24 0.50 5 92050 E32 A31 1.9 0.26 0.51 5 920 50 E33 A31 8.7 0.25 0.49 5 920 50 E34 A318.3 0.24 0.51 5 920 50 E35 A31 8.8 0.23 0.52 5 920 50

TABLE 14 Coated steel member Maximum C Maximum C Maximum C content insurface Maximum Cr content in high Al content in low Al layer area ofsteel content in high Al Tensile Bending Impact Steel content regioncontent region sheet substrate content region strength angle valueSymbol No. (mass %) (mass %) (mass %) (mass %) (MPa) (°) (J/cm²)Invention E1 A27 0.05 0.09 0.19 0.20 1712 70 52 Example E2 A27 0.05 0.100.18 0.19 1710 71 51 E3 A27 0.04 0.08 0.18 0.33 1703 77 53 E4 A27 0.050.07 0.20 0.34 1704 77 53 E5 A27 0.05 0.08 0.20 0.33 1711 71 53 E6 A270.04 0.08 0.19 0.35 1712 70 54 E7 A27 0.05 0.08 0.19 0.33 1710 70 52 E8A27 0.04 0.08 0.20 0.34 1708 71 52 E9 A27 0.04 0.07 0.19 0.35 1709 72 52E10 A27 0.03 0.07 0.17 0.33 1696 81 54 E11 A27 0.03 0.06 0.18 0.33 169980 53 E12 A28 0.07 0.10 0.21 0.08 1763 66 48 E13 A28 0.05 0.09 0.20 0.131758 69 48 E14 A28 0.06 0.09 0.22 0.12 1765 67 47 E15 A28 0.07 0.09 0.220.12 1765 66 47 E16 A28 0.06 0.09 0.21 0.13 1768 68 47 E17 A28 0.04 0.080.19 0.12 1755 72 49 E18 A29 0.08 0.12 0.25 0.07 1968 62 39 E19 A29 0.060.11 0.24 0.11 1961 64 40 E20 A29 0.07 0.12 0.26 0.11 1970 62 40 E21 A290.07 0.12 0.25 0.12 1968 62 40 E22 A29 0.07 0.11 0.24 0.12 1962 63 40E23 A29 0.05 0.10 0.23 0.11 1959 66 40 E24 A30 0.11 0.18 0.38 0.07 264255 26 E25 A30 0.08 0.17 0.36 0.11 2639 57 27 E26 A30 0.10 0.19 0.38 0.122644 55 25 E27 A30 0.11 0.19 0.37 0.12 2640 55 26 E28 A30 0.10 0.19 0.360.13 2650 56 27 E29 A30 0.07 0.16 0.36 0.11 2635 59 27 E30 A31 0.08 0.120.26 0.19 1749 66 47 E31 A31 0.06 0.11 0.24 0.34 1760 68 47 E32 A31 0.080.12 0.26 0.34 1751 65 48 E33 A31 0.07 0.12 0.25 0.35 1755 66 47 E34 A310.06 0.11 0.24 0.36 1750 67 47 E35 A31 0.05 0.10 0.23 0.34 1752 69 47

TABLE 15 Coated steel sheet Production conditions Maximum C content Heattreatment Amount of Si + Ni in surface layer Cr content TemperatureHeating Cooling rate to Ms Steel in upper layer of steel sheet in lowerlayer rising rate temperature or lower Symbol No. (mass %) (mass %)(mass %) (° C./s) (° C.) (° C./s) Comparative e1 A27 8.7 0.20 0.35 0.51150 50 Example e2 A27 10.2 0.19 0.34 0.5 1150 50 e3 A27 2.0 0.18 0.510.5 1150 50 e4 A27 1.2 0.17 0.50 0.5 1150 50 e5 A27 2.1 0.19 0.52 0.51150 50 e6 A27 1.1 0.20 0.54 0.5 1150 50 e7 A27 8.8 0.20 0.50 0.5 115050 e8 A27 10.8 0.19 0.48 0.5 1150 50 e9 A27 8.8 0.17 0.50 0.5 1150 50e10 A27 8.9 0.17 0.52 0.5 1150 50 e11 A27 8.9 0.17 0.52 5 740 50 e12 A278.9 0.17 0.52 5 1200 50 e13 A27 8.9 0.17 0.52 5 920 1 e14 A28 8.9 0.210.16 0.5 1150 50 e15 A28 2.1 0.20 0.25 0.5 1150 50 e16 A28 2.3 0.22 0.250.5 1150 50 e17 A28 8.8 0.22 0.23 0.5 1150 50 e18 A28 9.0 0.19 0.23 0.51150 50 e19 A28 9.0 0.19 0.23 5 740 50 e20 A28 9.0 0.19 0.23 5 1200 50e21 A28 9.0 0.19 0.23 5 920 1 e22 A29 8.9 0.26 0.12 0.5 1150 50 e23 A292.2 0.24 0.24 0.5 1150 50 e24 A29 2.0 0.25 0.25 0.5 1150 50 e25 A29 8.80.25 0.22 0.5 1150 50 e26 A29 8.9 0.23 0.21 0.5 1150 50 e27 A29 8.9 0.230.21 5 740 50 e28 A29 8.9 0.23 0.21 5 1200 50 e29 A29 8.9 0.23 0.21 5920 1 e30 A30 8.8 0.38 0.11 0.5 1150 50 e31 A30 2.1 0.36 0.23 0.5 115050 e32 A30 1.9 0.38 0.24 0.5 1150 50 e33 A30 8.8 0.37 0.22 0.5 1150 50e34 A30 8.7 0.35 0.21 0.5 1150 50 e35 A30 8.7 0.35 0.21 5 740 50 e36 A308.7 0.35 0.21 5 1200 50 e37 A30 8.7 0.35 0.21 5 920 1 e38 A31 8.7 0.260.33 0.5 1150 50 e39 A31 2.1 0.24 0.50 0.5 1150 50 e40 A31 1.9 0.26 0.510.5 1150 50 e41 A31 8.7 0.25 0.49 0.5 1150 50 e42 A31 8.3 0.24 0.51 0.51150 50 e43 A31 8.8 0.23 0.52 5 740 50 e44 A31 8.8 0.23 0.52 5 1200 50e45 A31 8.8 0.23 0.52 5 920 1

TABLE 16 Coated steel member Maximum C Maximum C Maximum C content insurface Maximum Cr content in high Al content in low Al layer area ofsteel content in high Al Tensile Bending Impact Steel content regioncontent region sheet substrate content region strength angle valueSymbol No. (mass %) (mass %) (mass %) (mass %) (MPa) (°) (J/cm²)Comparative e1 A27 0.15 0.17 0.28 0.22 1682 50 43 Example e2 A27 0.150.17 0.27 0.20 1683 49 45 e3 A27 0.14 0.16 0.28 0.35 1688 49 43 e4 A270.15 0.17 0.28 0.34 1682 50 44 e5 A27 0.14 0.16 0.27 0.34 1683 49 44 e6A27 0.14 0.17 0.27 0.36 1681 48 43 e7 A27 0.15 0.17 0.27 0.35 1689 49 45e8 A27 0.15 0.17 0.28 0.35 1689 49 43 e9 A27 0.15 0.16 0.27 0.35 1688 5044 e10 A27 0.14 0.16 0.27 0.35 1690 50 46 e11 A27 0.03 0.05 0.17 0.31876 89 91 e12 A27 0.14 0.16 0.27 0.36 1659 51 45 e13 A27 0.03 0.07 0.180.33 642 100 105 e14 A28 0.16 0.19 0.29 0.10 1752 48 42 e15 A28 0.150.19 0.29 0.12 1754 49 41 e16 A28 0.16 0.18 0.29 0.12 1753 48 42 e17 A280.15 0.18 0.30 0.12 1754 47 43 e18 A28 0.15 0.18 0.29 0.12 1753 48 41e19 A28 0.04 0.06 0.18 0.11 901 87 87 e20 A28 0.15 0.18 0.30 0.13 172049 42 e21 A28 0.04 0.08 0.19 0.11 689 99 103 e22 A29 0.18 0.21 0.34 0.091946 43 34 e23 A29 0.18 0.20 0.34 0.11 1948 43 33 e24 A29 0.17 0.21 0.350.12 1945 42 34 e25 A29 0.18 0.20 0.34 0.13 1943 43 33 e26 A29 0.17 0.200.34 0.12 1942 43 34 e27 A29 0.05 0.09 0.23 0.10 932 87 83 e28 A29 0.170.21 0.35 0.13 1911 44 34 e29 A29 0.05 0.11 0.24 0.11 710 92 97 e30 A300.26 0.33 0.49 0.09 2618 37 17 e31 A30 0.25 0.32 0.49 0.12 2619 37 15e32 A30 0.25 0.33 0.49 0.12 2620 36 16 e33 A30 0.25 0.32 0.49 0.12 261236 16 e34 A30 0.25 0.32 0.49 0.12 2617 37 17 e35 A30 0.06 0.15 0.35 0.101081 85 80 e36 A30 0.24 0.32 0.49 0.13 2578 38 17 e37 A30 0.07 0.17 0.360.11 842 89 91 e38 A31 0.18 0.21 0.34 0.23 1965 41 31 e39 A31 0.18 0.210.34 0.35 1968 41 30 e40 A31 0.18 0.21 0.34 0.34 1965 40 31 e41 A31 0.180.20 0.34 0.35 1963 41 30 e42 A31 0.17 0.21 0.34 0.35 1959 40 30 e43 A310.05 0.10 0.24 0.32 950 85 80 e44 A31 0.18 0.21 0.35 0.36 1932 42 32 e45A31 0.05 0.10 0.24 0.33 743 90 96

As can be seen from Tables 13 to 16, Invention Examples E1 to E35satisfying the ranges of the present invention obtained good results interms of both the structure and properties, but Comparative Examples e1to e45 not satisfying the ranges of the present invention obtainedresults not satisfying at least one of the structure and properties.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to obtain a highstrength coated steel member and a steel sheet having excellentcollision characteristics. The coated steel member according to thepresent invention is particularly suitable for use as a frame componentof a vehicle.

BRIEF DESCRIPTION OF THE REFERENCE SYMBOLS

1 Coated steel member

11 Steel sheet substrate

12 Al—Fe-based coating

121 Low Al content region

122 High Al content region

2 Coated steel sheet

21 Steel sheet

22 Al-based coating

221 Lower layer

222 Upper layer

The invention claimed is:
 1. A coated steel member comprising: a steelsheet substrate containing, as a chemical composition, by mass %, C:0.25% to 0.65%, Si: 0.10% to 2.00%, Mn: 0.30% to 3.00%, P: 0.050% orless, S: 0.0100% or less, N: 0.010% or less, Ti: 0.010% to 0.100%, B:0.0005% to 0.0100%, Nb: 0.02% to 0.10%, Mo: 0% to 1.00%, Cu: 0% to1.00%, Cr: 0% to 1.00%, Ni: 0% to 1.00%, V: 0% to 1.00%, Ca: 0% to0.010%, Al: 0% to 1.00%, Sn: 0% to 1.00%, W: 0% to 1.00%, Sb: 0% to1.00%, REM: 0% to 0.30%, and a remainder of Fe and impurities; and acoating formed on a surface of the steel sheet substrate and containingAl and Fe, wherein the coating has a low Al content region having an Alcontent of 3 mass % or more and less than 30 mass % and a high Alcontent region formed on a side closer to a surface than the low Alcontent region and having an Al content of 30 mass % or more, a maximumC content of the high Al content region is 25% or less of a C content ofthe steel sheet substrate, a maximum C content of the low Al contentregion is 40% or less of the C content of the steel sheet substrate, anda maximum C content in a range from an interface between the steel sheetsubstrate and the coating to a depth of 10 μm on a side of the steelsheet substrate is 80% or less of the C content of the steel sheetsubstrate.
 2. The coated steel member according to claim 1, wherein thesteel sheet substrate contains, as the chemical composition, Cr: 0.05%to 1.00%, and a maximum Cr content in the high Al content region is 80%or more of a Cr content of the steel sheet substrate.
 3. A method forproducing a coated steel member according to claim 1, the methodcomprising: a slab preparation step of melting a steel containing, as achemical composition, by mass %, C: 0.25% to 0.65%, Si: 0.10% to 2.00%,Mn: 0.30% to 3.00%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% orless, Ti: 0.010% to 0.100%, B: 0.0005% to 0.0100%, Nb: 0.02% to 0.10%,Mo: 0% to 1.00%, Cu: 0% to 1.00%, Cr: 0% to 1.00%, Ni: 0% to 1.00%, V:0% to 1.00%, Ca: 0% to 0.010%, Al: 0% to 1.00%, Sn: 0% to 1.00%, W: 0%to 1.00%, Sb: 0% to 1.00%, REM: 0% to 0.30%, and a remainder of Fe andimpurities, and casting to obtain a slab; a hot rolling step ofhot-rolling the slab to obtain a hot-rolled steel sheet; a coiling stepof coiling the hot-rolled steel sheet; a hot-rolled sheet annealing stepof annealing the hot-rolled steel sheet after the coiling in anatmosphere containing 80% or more of nitrogen at 450° C. to 800° C. for5 hours or longer; as necessary, a cold rolling step of descaling thehot-rolled steel sheet and cold-rolling the hot-rolled steel sheet toobtain a cold-rolled steel sheet; as necessary, an annealing step ofannealing the hot-rolled steel sheet or the cold-rolled steel sheet toobtain an annealed steel sheet; a coating step of forming an Al-basedcoating on the hot-rolled steel sheet, the cold-rolled steel sheet, orthe annealed steel sheet to obtain a coated steel sheet; and a heattreatment step of heating the coated steel sheet to an Ac₃ point to (Ac₃point+300)° C. at a temperature rising rate of 1.0 to 1000° C./s, andthereafter cooling the coated steel sheet to an Ms point or lower at anupper critical cooling rate or more.
 4. A method for producing a coatedsteel member according to claim 1, the method comprising: a slabpreparation step of melting a steel containing, as a chemicalcomposition, by mass %, C: 0.25% to 0.65%, Si: 0.10% to 2.00%, Mn: 0.30%to 3.00%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, Ti:0.010% to 0.100%, B: 0.0005% to 0.0100%, Nb: 0.02% to 0.10%, Mo: 0% to1.00%, Cu: 0% to 1.00%, Cr: 0% to 1.00%, Ni: 0% to 1.00%, V: 0% to1.00%, Ca: 0% to 0.010%, Al: 0% to 1.00%, Sn: 0% to 1.00%, W: 0% to1.00%, Sb: 0% to 1.00%, REM: 0% to 0.30%, and a remainder of Fe andimpurities, and casting to obtain a slab; a hot rolling step ofhot-rolling the slab to obtain a hot-rolled steel sheet; a coiling stepof coiling the hot-rolled steel sheet; as necessary, annealing thehot-rolled steel sheet; as necessary, a cold rolling step of descalingthe hot-rolled steel sheet and cold-rolling the hot-rolled steel sheetto obtain a cold-rolled steel sheet; an annealing step of annealing thehot-rolled steel sheet or the cold-rolled steel sheet to obtain anannealed steel sheet in an atmosphere having a dew point of 1° C. orhigher and in a temperature range of 700° C. to 950° C.; a coating stepof forming an Al-based coating on a surface of the annealed steel sheetto obtain a coated steel sheet by immersing the annealed steel sheet ina plating bath containing Si and Ni in a total amount of 7.0 to 30.0mass % and a remainder of Al and impurities; and a heat treatment stepof heating the coated steel sheet to an Ac₃ point to (Ac₃ point+300)° C.at a temperature rising rate of 1.0 to 1000° C./s, and thereaftercooling the coated steel sheet to an Ms point or lower at an uppercritical cooling rate or more.
 5. A method for producing a coated steelmember according to claim 1, the method comprising: a slab preparationstep of melting a steel containing, as a chemical composition, by mass%, C: 0.25% to 0.65%, Si: 0.10% to 2.00%, Mn: 0.30% to 3.00%, P: 0.050%or less, S: 0.0100% or less, N: 0.010% or less, Ti: 0.010% to 0.100%, B:0.0005% to 0.0100%, Nb: 0.02% to 0.10%, Mo: 0% to 1.00%, Cu: 0% to1.00%, Cr: 0% to 1.00%, Ni: 0% to 1.00%, V: 0% to 1.00%, Ca: 0% to0.010%, Al: 0% to 1.00%, Sn: 0% to 1.00%, W: 0% to 1.00%, Sb: 0% to1.00%, REM: 0% to 0.30%, and a remainder of Fe and impurities, andcasting to obtain a slab; a hot rolling step of hot-rolling the slab toobtain a hot-rolled steel sheet; a coiling step of coiling thehot-rolled steel sheet; a hot-rolled sheet annealing step of annealingthe hot-rolled steel sheet after the coiling in an atmosphere containing80% or more of nitrogen at 450° C. to 800° C. for 5 hours or longer; asnecessary, a cold rolling step of descaling the hot-rolled steel sheetand cold-rolling the hot-rolled steel sheet to obtain a cold-rolledsteel sheet; an annealing step of annealing the hot-rolled steel sheetor the cold-rolled steel sheet to obtain an annealed steel sheet in anatmosphere having a dew point of 1° C. or higher and in a temperaturerange of 700° C. to 950° C.; a coating step of forming an Al-basedcoating on a surface of the annealed steel sheet to obtain a coatedsteel sheet by immersing the annealed steel sheet in a plating bathcontaining Si and Ni in a total amount of 7.0 to 30.0 mass % and aremainder of Al and impurities; and a heat treatment step of heating thecoated steel sheet to an Ac₃ point to (Ac₃ point+300)° C. at atemperature rising rate of 1.0 to 1000° C./s, and thereafter cooling thecoated steel sheet to an Ms point or lower at an upper critical coolingrate or more.